Not sure if you've figured this out yet but I've found a fix, albeit a time consuming one IF you have a ton of songs/albums impacted by this issue. I subscribe to iTunes Music and very, very rarely purchase an album.
I will lay out what happened to me.
-Purchased an album (DMB Live 25) from outside iTunes as it wasn't available in iTunes and not sure if it ever will be.
-Imported album into iTunes with the correct Artwork.
-Turned off iCloud Music Library and transferred album to my iPhone via iTunes (that's a whole other story).
-Once album was on iPhone turned back on iCloud Music Library and chose to keep and merge.
-Started playing album and artwork for every song was different, even though it was correct in iTunes.
-I concluded (right or wrong) iTunes was pushing art work it thought was right for each song. I'm OCD and hated this.
After reading many posts I did the following. Remember, this was not labor intensive for me as the album is only 25 songs, but I can see this being a major pain for anyone with hundreds, thousands of songs.
-Deleted album from phone. Turned off iCloud Music Library on phone.
-Went into iTunes and edited each song title. I simply added "- 25" to end of each song. E.g. "Seek Up - 25"
-Instead of iTunes pulling the original album cover for the song "Seek Up" it now uses the DMB Live 25 artwork that came with the album.
-Re-downloaded the album to my iPhone.
-Turned back on iTunes Music Library and selected Keep and Merge and I'm good to go.
Good luck all.
Mike