Since updating to iOS 12 on my iPhone 6 I’ve had a major headache. Album art hasn’t been working for my iTunes match (iCloud) music library, and I’ve only been seeing the standard iTunes missing artwork icon for my songs & albums.
I even re-subscribed to Apple Music to see if the problem was iTunes match only, but it also affected most of the music I streamed from Apple Music too.
I’ve managed to find a really simple fix for the album artwork problem.
Head over into settings and click on the music tab. Once there, you need to disable iCloud music & Apple Music by clicking the toggles. Immediately after disabling iTunes Match (iCloud Music) and Apple music, you need to restart your iOS device. Do this as soon as you disable the toggles, that way you will stop iOS pruning off corrupted artwork files.
Once restarted, you can re-enable both the iCloud Music and Apple Music toggles in settings. At this point, it may ask if you want to delete music stored on your device & replace it with that in iCloud. Click yes at this point to force iOS to do a hard reset of the music XML file and cached thumbnails on the device. In this way iOS will delete all previous traces of the corrupted music database file and thumbnails.
When you open music, you should see the Apple Music app slowly start to cache your library & thumbnails again.
This took me a few days to fully rectify but going through the above steps seems to be the only way to force a hard reset of the apple music and icloud music cache.
Hope this helps to fix your broken album artwork in iOS 12.