Have had this issue since I upgraded to iPhone 12 Pro, from iPhone 10, along with the duplicated music issue (which I resolved by deleting all of the music on my phone in the storage section, then re-synching).
However, have found that viewing the album artwork in the 'On My Device' section of the iPhone page in iTunes shows the artwork ( for those tracks showing incorrect artwork on my phone ) as being blank, even though the artwork exists in the larger iTunes library.
If I then add the artwork to the tracks where it's missing, on the 'On My Device' screen, it then synch's the correct artwork to my phone.
No idea why these different places would be different though, considering the files were synch'ed from the same library.
Any one from Apple like to assist?