iTunes has been an absolute pig at handling album artwork for years, but 12.4 seems to have totally broken it. I keep a pretty tight ship with my music collection and everything is carefully curated. Album artwork has always been a struggle but I learnt how to work around it's horrendous problems. I use iTunes Match. Apple played around with the album artwork options under 12.4, for example under songs view there is no longer an option to right click a song and Get Album Artwork from the iTunes store (tip: you have to go into the Album view and do it from there). But now I can't delete album artwork. As soon as I do iTunes very quickly downloads the artwork I just deleted, so I cannot have a blank album cover (auto download album artwork is not enabled). It used to be that you would have to wait for a while for iTunes to update the (blank) cover in the cloud, but it would eventually happen. If you have iTunes Match, try deleting some album artwork and tell me what you see.