I'm not sure why MC can't report there is no actual cover art as that is the true state.
These files were pointing to some file other than folder.jpg and obviously the art is not embedded in tags.
Can the MC filter be set to double-check the file for embedded art?
yes . There is a difference though between not being associated with a file at all, and not being embedded, both should be corrected
the first expression will check for missing artwork (not associated, not in the file folder)
[Media Type]=[Audio] [Image File]=[] ~sort=[Album Artist],[Album]
The following is better because it will check to see if its embedded and missing. If you set one of the columns to show a little thumbnail, the ones with "associated" art will show up (if you have the field [Image File] to show up in your tag window it will show folder.jpg or whatever you have named the file.
(you can import this to a smartlist using the import/export button if you want)
[Media Type]=[Audio] -[Image File]=[Inside File] ~sort=[Album Artist],[Album]
If the file exists, but isn't "embedded", you can use the library tool Cover art=>Quick find in file folder/Cover art directory
If the file doesn't exist, but is embedded, you can use the library tool Cover art=> Save Cover art ....
If you have lots of these, its a good idea, imo, to rebuild your thumbnails afterwards