Did you get any solution for a downgrade? I myself have the same issue and no clue how to downgrade and nobody seems to be able to help or fix the issue
Using the UI, and after upgrading to HA 2021.9.6, I was able to go to Supervisor, Backups and click on an older version of Appdaemon addon backup that I wanted to downgrade to. The restore to this older version (kinda) worked as the version number was confirmed in the Appdaemon Info Page. However I wasn’t able to make the addon start (Authentiation error) but this is another issue altogether.