There are a couple of issues going on here. The first is that recent builds of ffmpeg (which Plex uses for reading this information from mp3 files) has a bug where it only grabs the first genre it sees, if the embedded genres are separated by null characters (the latest ID3 standard). If they are separated by semicolons, ffmpeg/Plex will grab all of the genres. This problem seems to exist only for .mp3 files. It isn’t a problem in .flac files, for instance. There is a fix in the ffmpeg pipeline for this, but who knows when it will land in a stable release. It apparently isn’t in the just-released version.
The second is that Plex doesn’t support the concept of track-level genres (there’s a feature request about this). For albums, it grabs the genres of the first track in the album and applies those tags to the album. This is something that Plex is reportedly working on changing.
There’s a fairly long discussion of these issues here, if you’re interested in reading the history:
