Had this exact problem. Have had iPhone 12 for 2 months, and finally synced it with USB cord to update a few playlists. After, half the songs had the wrong music album covers. 4 hrs of Apple online tech support and phone call later (after updating to Big Sur 11.4), tech help amounted to: delete all your music from phone, attempt sync again (this is for 17GB of music). Ended tech support and did the following solo: first, backup your library & playlists on an external drive and export your playlists just in case something goes wrong. If you have thousands of songs on playlists (some from old CDs, too) like I do, just do this step for peace of mind. Second, with the phone not connected to your computer, get rid of all the music on your phone: go to Settings > General > iPhone Storage > Music and swipe on All Songs and hit Delete. Then go to the Music app and delete all the playlists, so you see no songs or playlists when you're done. Connect you phone to your computer, and uncheck Sync Music. Sync your phone to confirm that there are no songs or playlists that are synchronizing. Check Sync Music, and test one playlist to make sure it works. If it does, sync the rest. If anything goes wrong during this process, no worries, you have your backup music/playlists on the external drive. Note: upgrading to Big Sur likely had no effect on this issue and was an unnecessary step, because the problem persisted afterward. My guess is that there is a database issue between my old phone, new phone, and iTunes, so that the problem only showed up once I tried to sync the new phone to my laptop. If that's the case, the problem has to be fixed by Apple to avoid the issue altogether.