I flashed another Pixel 3a XL this morning for a friend that I got interested in this, cause flashing misery loves company right? LOL He tried for two days but could not get Google Play Services to flash the phone back to 9.0.0 (PQ3B.190801.002, Aug 2019). I tried this morning and couldn't get it to work either, so I resorted to doing it manually and it went off without a hitch. Here is what I did. And just for the record, I'm on Linux, Ubuntu to be exact. You should have ADB and fastboot installed on your computer.
Go to the Google play services site and instead of "Flash" click on "Link" next to the 9.0.0 (PQ3B.190801.002, Aug 2019) image. It will download the zip. Unzip it and cd into it in a terminal.
Start with the phone unlocked and on in a regular operational state with developer mode and USB debugging enabled. Bounce (turn off and on) USB debugging so you get the pop-up to always allow from this computer.
In the terminal and run "adb devices" to make sure you can see it. then run "adb reboot-bootloader" which will take you to the fastboot screen.
The "Flash-all.sh" script from the zip you downloaded will fail, at least it always did for me, due to the "if" statement at the beginning. Instead, cat the script and just run the fastboot commands in it manually. There are only a few to run and remember you have to be in the directory these files are in to run the commands. This worked on two phones so far. I then ran the 0.8.8 installer and like magic, it was done. Like anything else in Linux, there are several ways to do anything and you never know when you might have to do something different for some unknown reason. I believe in picking up these referb phones, there is just no telling the how's or why's in which they were wiped and what it will take to get them to flash. But here's yet another way.