Most of my music was imported from CD and I have recently reimported all of it to be Apple Lossless. I don’t use Apple Music and I don’t buy a lot of songs from Apple. And I don’t use the iCloud music library.
I have spent a bit of time setting the Album Artist and making sure the Compilation checkbox is never used. A lot of times, the album artwork does not appear when I import an CD, so I end up going to google to find an image to use. I no longer use the Add Album Artwork function since I don’t want the random errors.
Thus, I’ve painstakingly checked to make sure the album artwork was there and correct. When I saw weird things on my iPhone, it was obvious something went wrong.
I’m not sure how all this works if you also use the iCloud Music Library. What is stored in the cloud? I think Apple only uploads your original music files if it can’t find a match in their extensive catalog. And I would think everything in their catalog that found a match has the correct album artwork.
When you use the cloud, is the music actually on your phone or are you essentially streaming it? I know with my Photos, all of my originals are in the cloud so if I don’t have internet I can’t watch my videos if they are not already on my phone (e.g. recently recorded w my phone).
I don’t know how adjusting metadata and album artwork works with the cloud since in most cases, you’ve been connected to Apple’s copy of the song. If you download for offline listening onto your phone (does Apple offer that?), it’s most likely downloading Apple’s files so they should have the artwork.
Could the same wackiness happen with their files? Can you go to a browser, go to iCloud and listen to your music in the cloud? Do you see issues with the artwork when you do? If the music files look better in the cloud than on your phone, maybe you have to turn off the iCloud music library on your phone so that it will redownload the music files when you turn it back on. I’m just guessing here so be careful.