@embeddedmz , did you try to follow this instructions?
https://forums.ubports.com/topic/6951/ubports-quick-and-simple-install-on-xiaomi-mi-a2

I was in same sittuation, phone I bought had Android 10 but I downgraded it following tutorial I linked above.

@TheBird , yes. I noticed same - I though however to look for information if anyone knows how to fix it without workaround like that.
I will live with that for now - thank you a lot.

This guide worked well for me.
Unfortunately I could not install Ubuntu at the end ( installer was getting <waiting for device> error due to fastboot crashing after downgrade to Andreoid 9). On some other articles people were suggesting it could be USB Port issue, that may be true actually. As I had no other idea I installed "installer" on other laptop (switched from Ubuntu to Mac) and there it worked..

@embeddedmz , not sure if root cause is same but I had same problem. On other chat I found some people saying issue could be in USB 3 port. I switched from my Ubuntu laptop (which I experience problem at same stage) to Mac from 2015 and ubports installer managed did the job there
not sure if that helps..

@embeddedmz you are correct. I see same after reading your reply.
I am not able to say if this problem came with recent ota-21 update as I did not use BT before but niw I,see same problem you describe (including toggle goin back to disabled)