Does the file have an embedded poster?
In the past, MP4s with embedded posters have given me trouble with posters not appearing. Could never nail down why this happens to some, but not all, MP4s. Could never tie it to poster size, type (jpg/gif/etc.), or anything else.
Workarounds:
Option: Tell Plex to ignore embedded metadata
Edit the library → Advanced → unselect “Prefer local metadata.” Save & refresh metadata for the movie. This affects all media in the library, not just the individual movie.
Option: Remove the embedded poster.
Use tools such as Mp3tag, Subler (Mac only), etc. to remove the poster from the MP4 container. Then Plex Dance the file.
Option: Remux to MKV.
Use MKVToolNix or similar tools to remux to a MKV container. Plex will ignore the embedded poster (you can also remove it when remuxing).
Option: Add poster file to folder with movie.
See Local Media Assets - Movies.
