The Flac files are a problem with Plex. Plex will read the embedded tags for each file and apply them to the album the tracks are from. Plex doesn’t store individual track genre.
Plex then applies the genres from the Album(s) to the artist. (Sometimes a refresh metadata is required at the Artist level) This works for all formats, except flac.
You should see all available genres in the “Albums” view, but if you use the artist view, genres from flac files will not be available.
It’s apparently a server bug, and apparently fixed in an upcoming release.
