Sometimes when I make some changes with backdrops or other stuff the bacdrops and collection images get messed up. A manual refresh with ctrl-R doesn’t make it a right and also a scheduled refresh is not helping. When deleting the database and build a new one makes it right again. I guess in that case it takes the backdrops from the Boxsets folder in the IBN instead of the movie(s) folder itself.
In this example we see the Pirates of the Caribbean with a collection image from Lethal weapon and a backdrop from Indiana Jones.

Hmmm I’ll have a look around to see if I can replicate, but I’ll need to copy your boxset structure. If you don’t mind can you post your Pirates of the Caribbean setup please?