Quick workaround
I too am experiencing this issue and I have a substantial library with perfectly sourced artwork.
One workaround I've found to restore the artwork is to 'Get Info' on the first track of an album and go to the 'Artwork' tab. You should see your artwork there and iTunes will immediately populate the grid view and all the other views with the correct artwork for that album. Kinda weird, but I hope this helps someone.
Pro tip: Create a smart playlist where it only has tracks whose track number is 1. View the playlist in grid view, use the arrow keys to navigate to busted covers and then command+i to open info, (it will auto open to artwork after the first one), and then hit return. You can blaze through them pretty quickly that way.