This will be fixed soon.
There was a regression in Plex when they switched to a new tag scanning library (ffmpeg), multivalued tags in id3v2.4 (and mp4/aac) are indeed not picked up anymore.
However, a patch has recently been submitted in ffmpeg that will fix it, so as soon as that’s accepted, it will cascade down to Plex.
See also: Only one embedded genre tag being read
