This is the output from /proc/mounts (though adb shell):

jasmine_sprout:/ $ cat /proc/mounts                                                                                         
/dev/root / ext4 ro,seclabel,nodev,relatime 0 0
tmpfs /dev tmpfs rw,seclabel,nosuid,relatime,size=1896744k,nr_inodes=474186,mode=755 0 0
devpts /dev/pts devpts rw,seclabel,relatime,mode=600 0 0
proc /proc proc rw,relatime,gid=3009,hidepid=2 0 0
sysfs /sys sysfs rw,seclabel,relatime 0 0
selinuxfs /sys/fs/selinux selinuxfs rw,relatime 0 0
tmpfs /mnt tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=1896744k,nr_inodes=474186,mode=755,gid=1000 0 0
tmpfs /apex tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=1896744k,nr_inodes=474186,mode=755 0 0
/dev/block/dm-1 /vendor ext4 ro,seclabel,relatime,discard 0 0
none /dev/cg2_bpf cgroup2 rw,nosuid,nodev,noexec,relatime 0 0
none /dev/cpuctl cgroup rw,nosuid,nodev,noexec,relatime,cpu 0 0
none /acct cgroup rw,nosuid,nodev,noexec,relatime,cpuacct 0 0
none /dev/cpuset cgroup rw,nosuid,nodev,noexec,relatime,cpuset,noprefix,release_agent=/sbin/cpuset_release_agent 0 0
none /dev/stune cgroup rw,nosuid,nodev,noexec,relatime,schedtune 0 0
/dev/root /apex/com.android.tzdata@290000000 ext4 ro,seclabel,relatime 0 0
/dev/root /apex/com.android.tzdata ext4 ro,seclabel,relatime 0 0
/dev/root /apex/com.android.runtime@1 ext4 ro,seclabel,relatime 0 0
/dev/root /apex/com.android.runtime ext4 ro,seclabel,relatime 0 0
debugfs /sys/kernel/debug debugfs rw,seclabel,relatime 0 0
none /config configfs rw,nosuid,nodev,noexec,relatime 0 0
pstore /sys/fs/pstore pstore rw,seclabel,nosuid,nodev,noexec,relatime 0 0
tracefs /sys/kernel/debug/tracing tracefs rw,seclabel,relatime 0 0
/dev/block/bootdevice/by-name/userdata /data ext4 rw,lazytime,seclabel,nosuid,nodev,noatime,discard,noauto_da_alloc,resgid=1065,data=ordered 0 0
/dev/block/bootdevice/by-name/modem_a /vendor/firmware_mnt vfat ro,context=u:object_r:firmware_file:s0,relatime,uid=1000,gid=1000,fmask=0337,dmask=0227,codepage=437,iocharset=iso8859-1,shortname=lower,errors=remount-ro 0 0
/dev/block/bootdevice/by-name/bluetooth_a /vendor/bt_firmware vfat ro,context=u:object_r:bt_firmware_file:s0,relatime,uid=1002,gid=3002,fmask=0337,dmask=0227,codepage=437,iocharset=iso8859-1,shortname=lower,errors=remount-ro 0 0
/dev/block/bootdevice/by-name/dsp_a /vendor/dsp ext4 ro,seclabel,nosuid,nodev,relatime,data=ordered 0 0
/dev/block/bootdevice/by-name/persist /mnt/vendor/persist ext4 rw,seclabel,nosuid,nodev,noatime,data=ordered 0 0
tmpfs /storage tmpfs rw,seclabel,nosuid,nodev,noexec,relatime,size=1896744k,nr_inodes=474186,mode=755,gid=1000 0 0
/dev/root /apex/com.android.conscrypt@290000000 ext4 ro,seclabel,nodev,relatime 0 0
/dev/root /apex/com.android.conscrypt ext4 ro,seclabel,nodev,relatime 0 0
/dev/root /apex/com.android.media@290000000 ext4 ro,seclabel,nodev,relatime 0 0
/dev/root /apex/com.android.media ext4 ro,seclabel,nodev,relatime 0 0
/dev/root /apex/com.android.media.swcodec@290000000 ext4 ro,seclabel,nodev,relatime 0 0
/dev/root /apex/com.android.media.swcodec ext4 ro,seclabel,nodev,relatime 0 0
/dev/root /apex/com.android.resolv@290000000 ext4 ro,seclabel,nodev,relatime 0 0
/dev/root /apex/com.android.resolv ext4 ro,seclabel,nodev,relatime 0 0
adb /dev/usb-ffs/adb functionfs rw,relatime 0 0
/data/media /mnt/runtime/default/emulated sdcardfs rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=1015,multiuser,mask=6,derive_gid,default_normal,unshared_obb 0 0
/data/media /storage/emulated sdcardfs rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=1015,multiuser,mask=6,derive_gid,default_normal,unshared_obb 0 0
/data/media /mnt/runtime/read/emulated sdcardfs rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,multiuser,mask=23,derive_gid,default_normal,unshared_obb 0 0
/data/media /mnt/runtime/write/emulated sdcardfs rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,multiuser,mask=7,derive_gid,default_normal,unshared_obb 0 0
/data/media /mnt/runtime/full/emulated sdcardfs rw,nosuid,nodev,noexec,noatime,fsuid=1023,fsgid=1023,gid=9997,multiuser,mask=7,derive_gid,default_normal,unshared_obb 0 0

jasmine_sprout:/ $ df
Filesystem            1K-blocks    Used Available Use% Mounted on
/dev/root               3047900 2544264    487252  84% /
tmpfs                   1896744    1016   1895728   1% /dev
tmpfs                   1896744       0   1896744   0% /mnt
tmpfs                   1896744       0   1896744   0% /apex
/dev/block/dm-1         2031964  667752   1347828  34% /vendor
/dev/block/mmcblk0p69  48110436  161536  47801444   1% /data
/data/media            48110436  161536  47801444   1% /storage/emulated

jasmine_sprout:/ $ ls -la /dev/block/by-name/                                                                                                                                                    
total 0
drwxr-xr-x 2 root root 1460 1972-03-30 04:01 .
drwxr-xr-x 6 root root 2120 1972-03-30 04:01 ..
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 abl_a -> /dev/block/mmcblk0p25
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 abl_b -> /dev/block/mmcblk0p26
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 apdp -> /dev/block/mmcblk0p11
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 bk1 -> /dev/block/mmcblk0p51
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 bluetooth_a -> /dev/block/mmcblk0p38
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 bluetooth_b -> /dev/block/mmcblk0p50
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 boot_a -> /dev/block/mmcblk0p62
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 boot_b -> /dev/block/mmcblk0p63
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 cache -> /dev/block/mmcblk0p64
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 cmnlib64_a -> /dev/block/mmcblk0p22
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 cmnlib64_b -> /dev/block/mmcblk0p24
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 cmnlib_a -> /dev/block/mmcblk0p21
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 cmnlib_b -> /dev/block/mmcblk0p23
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 ddr -> /dev/block/mmcblk0p28
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 devcfg_a -> /dev/block/mmcblk0p30
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 devcfg_b -> /dev/block/mmcblk0p31
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 devinfo -> /dev/block/mmcblk0p43
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 dip -> /dev/block/mmcblk0p27
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 dpo -> /dev/block/mmcblk0p2
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 dsp_a -> /dev/block/mmcblk0p48
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 dsp_b -> /dev/block/mmcblk0p49
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 frp -> /dev/block/mmcblk0p14
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 fsc -> /dev/block/mmcblk0p3
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 fsg -> /dev/block/mmcblk0p45
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 hyp_a -> /dev/block/mmcblk0p17
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 hyp_b -> /dev/block/mmcblk0p18
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 keymaster_a -> /dev/block/mmcblk0p36
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 keymaster_b -> /dev/block/mmcblk0p37
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 keystore -> /dev/block/mmcblk0p13
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 limits -> /dev/block/mmcblk0p4
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 logdump -> /dev/block/mmcblk0p57
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 logfs -> /dev/block/mmcblk0p44
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 mdtp_a -> /dev/block/mmcblk0p52
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 mdtp_b -> /dev/block/mmcblk0p53
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 mdtpsecapp_a -> /dev/block/mmcblk0p34
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 mdtpsecapp_b -> /dev/block/mmcblk0p35
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 minidump -> /dev/block/mmcblk0p61
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 misc -> /dev/block/mmcblk0p42
lrwxrwxrwx 1 root root   18 1972-03-30 04:01 mmcblk0 -> /dev/block/mmcblk0
lrwxrwxrwx 1 root root   22 1972-03-30 04:01 mmcblk0rpmb -> /dev/block/mmcblk0rpmb
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 modem_a -> /dev/block/mmcblk0p58
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 modem_b -> /dev/block/mmcblk0p59
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 modemst1 -> /dev/block/mmcblk0p46
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 modemst2 -> /dev/block/mmcblk0p47
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 msadp -> /dev/block/mmcblk0p12
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 oops -> /dev/block/mmcblk0p10
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 persist -> /dev/block/mmcblk0p55
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 persistbak -> /dev/block/mmcblk0p56
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 pmic_a -> /dev/block/mmcblk0p19
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 pmic_b -> /dev/block/mmcblk0p20
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 rawdump -> /dev/block/mmcblk0p60
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 rpm_a -> /dev/block/mmcblk0p15
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 rpm_b -> /dev/block/mmcblk0p16
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 sec -> /dev/block/mmcblk0p6
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 splash -> /dev/block/mmcblk0p54
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 ssd -> /dev/block/mmcblk0p5
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 sti -> /dev/block/mmcblk0p39
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 storsec -> /dev/block/mmcblk0p9
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 switch -> /dev/block/mmcblk0p1
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 system_a -> /dev/block/mmcblk0p65
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 system_b -> /dev/block/mmcblk0p66
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 toolsfv -> /dev/block/mmcblk0p29
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 tz_a -> /dev/block/mmcblk0p32
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 tz_b -> /dev/block/mmcblk0p33
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 userdata -> /dev/block/mmcblk0p69
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 vbmeta_a -> /dev/block/mmcblk0p7
lrwxrwxrwx 1 root root   20 1972-03-30 04:01 vbmeta_b -> /dev/block/mmcblk0p8
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 vendor_a -> /dev/block/mmcblk0p67
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 vendor_b -> /dev/block/mmcblk0p68
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 xbl_a -> /dev/block/mmcblk0p40
lrwxrwxrwx 1 root root   21 1972-03-30 04:01 xbl_b -> /dev/block/mmcblk0p41

I don't know if this is informative. The phone is not rooted so I don't have permission to /proc/partitions.

From this, here is my process:

(base) peter@pt450:~/Downloads$ fastboot boot twrp.img
downloading 'boot.img'...
OKAY [  0.780s]
booting...
OKAY [  0.155s]
finished. total time: 0.935s

jasmine_sprout_stock_android9.zip: 1 file pushed. 28.6 MB/s (1131322942 bytes in 37.715s)

In TWRP, select this zip for installation.

Updating partition detail…
...done
Full SELinux support is present.
MTP Enabled
Installing zip file ‘/jasmine_sprout_stock_android9.zip’
Checkeing for Digest file…
Unmounting System…
Flashing A/B zip to inactive slot: B
Step 1/2
Step 2/2
To flash additional zips, please reboot recovery to switch to the updated slot.
Updating partition details…
...done

“Reboot System” in TWRP. This boots into Android 10 (which resides in Slot A).

peter@pt450:~/Downloads$ adb reboot fastboot
peter@pt450:~/Downloads$ fastboot set_active b
Setting current slot to 'b'...
OKAY [  0.026s]
finished. total time: 0.026s
peter@pt450:~/Downloads$ fastboot reboot
rebooting...

finished. total time: 0.050s

The system doesn’t boot up. I get this on my phone:

Android Recovery
xiaomi/jasmine/jasmine_sprout
9/PKQ1.180904.001/V10.0.2.0.PDIMIFJ
user/release-keys
Use volume up/down and power.
Can’t load Android system. Your data may be corrupt. If you continue to get this message, you may need to perform a factory data reset and erase all user data stored on this device
[Try again][Factory data reset]

xiaomi’s V10 is in fact Android 9; so this is the correct version. At this point, Try again doesn’t work (i.e. boots to the same message), and I go for the factory data reset. After that, a fresh Android 9 greets me. I go through the initial settings, then I enable USB degugging. I restart Android one more time just to be sure.

peter@pt450:~/Downloads$ adb devices
List of devices attached
8542b8c	unauthorized
peter@pt450:~/Downloads$ adb reboot fastboot
error: device unauthorized.
This adb server's $ADB_VENDOR_KEYS is not set
Try 'adb kill-server' if that seems wrong.
Otherwise check for a confirmation dialog on your device.

The odd thing is, on the phone no authorization dialogue popped up when I enabled USB debugging. I revoke the USB debugging authorizations on the phone, disable and re-enable USB debugging, and now the authorization dialogue does pop up.

peter@pt450:~/Downloads$ adb reboot fastboot

This does not boot into fastboot mode, but into the android image. I reboot into fastboot mode with the Power+Vol_down key combination.

peter@pt450:~/Downloads$ fastboot set_active b
error: Device does not support slots.
peter@pt450:~/Downloads$ fastboot boot twrp.img
downloading 'boot.img'… #this stalls, nothing happens
peter@pt450:~/Downloads$ fastboot reboot #this stalls, nothing happens

So, I’m in a state with a non-working fastboot again. I can’t even fastboot reboot (I thought this was still possible the last time, but I might be wrong). I reboot by holding the Power button for some seconds, and I can boot into android 9 again.

I guess I will have to work my way through the internal updates again, to get back to android 10 with working fastboot. I have the appropriate zip for android 10, but I don't see the way to get in on the device...

I guess I will have to work my way through the internal updates again, to get back to android 10 with working fastboot. I have the appropriate zip for android 10, but I don't see the way to get in on the device...

Off-topic: I couldn't edit my message to append these two sentences, since akismet flagged it as spam????

@AppLee As a user I will not troubleshoot the source code (which is also written in a language I don't speak). Where are the log files you speak of? They are not in the directory that ubports-installer is run from, they are not in $HOME. If they exist, that's great, but the existence and the location should be documented. The easiest solution is often to print everything to the terminal...
The Mi A2 is not a "new device" that should "require some work". I actually bought this device specifically since it is listed as supported by the installer and it is listed as a device where "everything" works.
Don't get me wrong, I'm aware that this is a community project. And I'm further aware that the Mi A2 is a community device. It's not a problem if things don't work. However, UBports presents itself (on ubports.com) as a project that is hiding a lot of technical details from its target audience, thereby making the impression that it's not necessary to care about technical details. "Just run the installer and go." Not.
@belohoub Thanks, that is valuable information. With the latest installer (0.8.7), I tried again to install the stable channel. But it hangs at "Cleaning Up - Formatting system partition" with no further information on the shell, just a lot of CPU load. Or does one have to wait (hours?) long? Afterwards the device is again in a state where fastboot is broken. Seems like I have to go the lengthy upgrade path to android 10 again to get a working device. I might try to re-flash the android 9-image once more and try again. At some point it will be time to settle for Lineage OS