Just changing the folder which the movie is in resulted in the same behaviour. Poster changed and background missing. Checking the xml you can clearly see this and also see that background is missing.
Before:
<Image alt="Arthur and the Invisibles" type="coverPoster" url="/library/metadata/413195/thumb/1744796956"/>
<Image alt="Arthur and the Invisibles" type="background" url="/library/metadata/413195/art/1744796956"/>
<Image alt="Arthur and the Invisibles" type="clearLogo" url="/library/metadata/413195/clearLogo/1744796956"/>
After:
<Image alt="Arthur and the Invisibles" type="coverPoster" url="/library/metadata/413216/thumb/1744796956"/>
<Image alt="Arthur and the Invisibles" type="clearLogo" url="/library/metadata/413216/clearLogo/1744796956"/>
I think it’s the changing of the item used in matching which is important. For me, it’s the folder but for you it might be file depending on how you have setup your media & library.
Does that matter ? The movie is matched correctly and plex should map it and use the artwork and/or local assets associated with the entry. These are broken by this scenario.
Again, folks are trying to find repeatable steps on why artwork is changing. Here it is. Now plex need to go fix this.