This just started happening the other day. Some poster art started to turn into a white blank. This is present in the library, in my favorites folder, on the favorites bar, and the ATV home screen. The only place I can see the actual movie poster is when I click on the movie and you get the movie synopsis, where you hit play. The poster is still white on the bottom part, though.
I use both local and online metadata for different folders and this issue seems to be indiscriminate between the two. Moving the movie doesn’t do anything. I’ve removed it from the folder, refreshed metadata, then added it back, but the problem is still there.