I would try doing the “Plex Dance” for Music with the album. (The clean bundles step can be skipped)
- Move the album out of the library, so that Plex can’t see it.
- If you have Plex configured to automatically see changes in your library, a trash can will show up on the album in a minute or two. If you don’t have it configured that way you will need to scan the library.
- Empty the trash from the library
- Put the album back into the library (re-scan if your system does not do it automatically)
Hopefully, the genres show up under the artist 
