I have had this problem for quite some time now, my movie posters in my library keep changing to framegrabs, all info is there when I edit the movie, even the posters.
Is this installation already a few years old?
And if so, did you perhaps manually select posters of the affected items?
And did you convert the library to use the new “Plex Movie” metadata agent?
If the answer is Yes to all of the above, you might want to unlock these old manual poster selections. They were probably set ot a poster which was available through the old agent, but is no longer there through the new agent
Trigger a Refresh Metadata of the whole libaray after the procedure.
Please inspect the Plex media info XML of affected items.
Look for the first occurrence of the XML property guid="... (with all lower case letters) and note down its contents.
Then look at the properties of the library and tell what is selected for Agent on the Advanced tab.