genre trouble

[MichaelSaunders5]

In my Plex audiobook library, I have:

Agent = Audnexus Agents
Keep existing genres - CHECK
Prefer Local Metadata - UNCHECK
Genres = None

In my .m4b files I have MP4 GENRE set with semicolon-separated tags, e.g.:

GENRE = "fiction;literature"

I scan the book or refresh metadata. Everything else appears to work but Plex's genre tags are not set. The Plex genre field remains empty. In the above example, I would expect to see "fiction" and "literature" in this field. This is what happens when I use the Plex Music agent.

Am I doing something wrong or is this a bug? Thanks!