There used to be an option to select how many distinct genres you want Plex to show. I think the options were like 12, 75, and a million or something and, was found in the advanced section of editing the library but, I can’t find it anymore, anywhere I look.
I think those were the genres provided by Gracenote. Plex is no longer using Gracenote for metadata, so that’s probably why you’re not seeing them. Apparently, they have a new metadata provider in the works, and hopefully that lands soon.