Looks like the "folder.jpg" is missing from the album folder.
There are 2 artwork files, one is as you say embedded in the file, but the other named "folder.jpg" is in the album folder along with the tracks, I think it's this one that is either missing or incorrectly labeled.
Maybe this will help you to understand what I mean:-
https://www.youtube.com/watch?v=usGptonJw9MI've attached an image below..