I am not sure how this is working for you but I was able to replicate a permissions issue with Mono v6.4/v6.8. I tried 6.8 and 6.4 and both versions give a permissions error that sonarr/lidarr/radarr is unable to move files from the downloads folder after sab downloads them. I have had a standard jail setup for years working and only stopped after upgrading mono to v6. After switching back to Mono v5 I have no issues and the files are processed.I'm running Sonarr 22.214.171.12444 and Mono 126.96.36.199 (grabbed from comment #75) with no issues.
NOTE: I'm not running Sonarr "plugin"; I created the jail and installed everything manually, that's the way I like it.
This is on Truenas 12.0-Release and iocage jail is at Freebsd 12.1.