God knows why but plex consolidates the genre metadata from many sources (imdb, tmdb, tvdb and probably some other places) making it in essence useless. I’ve movies in my collection with 10+ genres assigned.
Just look at https://watch.plex.tv/movie/mulan as one example, its 11 
If you have any votes free then consider voting here [Feature Request] Ability to specify Metadata from a single source (IMDB, TMDB or TVDB).
