You may find this of interest: What MP4 tags can be read into Plex with local assets agent? - #11 by OttoKerner
as well as Picture on Cast actors not showing - #17 by OttoKerner
Make the configuration change mentioned in my previous post, then Refresh Metadata on the affected movies.
In Plex, this is only ever an issue with mp4/m4v files. Plex doesn’t read title and other tags from any other video container formats.
