Noticed a new bug since a few weeks ago.
For a box set like this
<Album 1> - Tracks
<Album 2> - Tracks
<Album 3> - Tracks
Cover.jpg
when you delete Cover.jpg in the parent folder and rescans the box set, it searches an image in sub-folders and replaces the deleted image with the first found one.
So if <Album 2> happens to contain folder.jpg, then now the entire tracks are linked to that image instead of the deleted Cover.jpg.