Wow, that's really strange. I'm not sure if will help, but you could try rebuilding thumbnails on a few files, just to see if it makes the art show back up. Right click on a file > Cover Art > Rebuild Thumbnail.
You could try doing a quick find on the art: Right click on a file > Cover Art > Quick Find in File / Cover art directory .
You could try finding the art manually by either using the Get From Internet option, or Add From File option. The later would be if you know where the art is, and you want to "rehook" it to the file.
I'm not suggesting that adding the art again to all of your files is a good solution. I'm suggesting those steps as troubleshooting. The last ones should show us if MC21 can show album art *at all*.
Good luck,
Brian.