After running mbsync on my library, I’ve got plenty of “mbsync: Skipping album with invalid mb_albumid:” logged. I think that I should remove invalid entries from database and files, maybe not only mb_albumid but all MusicBrainz tags and try to reimport them to identify again. How can I remove these invalid IDs from all affected tracks and albums?
This is just conjecture, but the mb id fields are also used by other autotaggers, so anything tagged by discogs, etc would result in ids that mb would likely consider invalid. So if you have other taggers in use, this probably isn’t actually a problem. Check the data_source field.
I don’t think there is a way to just detect invalid IDs, but do recall that you can list out the IDs and look at them yourself: beet ls -f '$albumartist - $album: $mb_albumid'