I’ve run into this problem with MP4 files that have embedded cover art. Never did come up with a good diagnosis or resolution. Here’s the thread: PMS not using embedded Cover Arts. Not sure if MKVs are also affected. I do not have MKVs with embedded artwork.
A further complication is that Plex sometimes cannot provide cover art due to legal reasons. See this post: Once Upon a Time in the West metadata - #4 by dane22. This applies to all media, irrespective of file type (MP4, MKV, etc).
A Plex Pass makes no difference.
Options / Workarounds
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Configure Plex to not look for embedded metadata.
This applies only to MP4 files. Plex does not read embedded metadata from other containers.
This affects all MP4 files in the library and applies to all embedded metadata, not just artwork.
Edit the Library → Advanced → deselect Prefer local metadata. -
Manually select cover art.
Edit the movie (pencil icon) and manually choose a poster.
Embedded cover art should be displayed.
Online cover art will not be displayed if restricted due to take down notices. -
Remove the embedded cover art from the MP4 container, then refresh metadata.
Plex will retrieve cover art, if available, from online sources. -
Add a separate picture file to the same folder as the movie.
Edit the Library → Advanced → enable Use Local Assets (on by default)
See Local Media Assets – Movies.
