I tried to complete the genre feature of an album from Jimmy McGriff ,The Worm’. After logging in, Picard told me that no one has tagged the album yet. So I went ahead and tagged it with ‘jazz’, ‘soul jazz’. The tags as such were accepted, but when looking up the album in MusicBrainz Picard, it showed on track 1 under Genre ‘Unknown’ ‘255’ and on track 2 still ‘Unknown’ ‘255’, yet the tags I added earlier on (jazz, soul jazz) appeared in the Genre tab (see attached screenshots)
The left column is the current value stored in the file, so you can ignore the ‘Unknown’ tag.
It looks like it’s pulling it from the ‘255’ tag that someone added to the release group:
I haven’t dug around genres much, but since ‘255’ isn’t on the MB genre list you should be able to go to Picard > Options > Metadata > Genres and untick ‘use folksonomy tags as genre’ and it wont tag with it anymore. I add all kinds of notes (tags) to releases that you wouldn’t want to have in your genre field.
Alternatively, you could downvote the 255 to remove it, I think. I have no idea what the tag means but it’s been used a bit so it might be legit: https://musicbrainz.org/tag/255
