No Linux guy here, so I can’t help with anything specific to that.
But if the library is quite old and/or has been converted from the old agents to Plex Movie, then you might have to perform Reset all manual poster selections in a library at once or unlock other metadata fields
edit: to verify that the above procedure is the right choice, look at the Plex media info XML of affected movies. If you find <Field locked="1" name="thumb"/> in there, this is a confirmation.
