Actually, something like 30% of my movies all of a sudden stopped showing their artwork, unless I brought up the info menu, then it would suddenly be there. Then when it was off screen as I scrolled away and came back, it was gone again.
This was something in excess of 300 movies. Also on the appletv nothing would show for the artwork.
Can't say I've looked in my music library so will have to check.
For the movies, I use iFlicks for the meta data tagging, and so selected all the movies that showed the issue and ran iFlicks through the services menu of iTunes redoing the metadata. Worked a treat and all the items are holding now.
Best I can offer.