iTunes album artwork not always synced to iPhone Music app

I use Match (not Music) and although songs, albums and playlists all sync almost right away on Music when I add or modify something in iTunes, the same doesn't happen for artwork. After hours, or days...even. So I don't think I must simply "give it time".

I've added many new albums on iTunes which synced fine on Music, but on Music there's no artwork! It bugs me a lot, I want to see my artwork when I listen on Music. If it's on iTunes, then I don't see why it wouldn't sync.

This happens both with iTunes downloaded artwork, and artwork I choose myself and manually add. So it's not even an "iTunes native artwork" issue. I really need to fix this. I hate that empty artwork on Music. Anyone having the same problem or a solution? Thank you!