I have 110,000 songs in my Library. I saw the Adding Artwork notice in the menu bar and was skeptical. But then I looked at the MAC HD info and saw that it adding files to the drive - slowly. I have my library on an external drive and the database for Music is on the local drive. I have set my mac to stay awake so it continues to add and left Music open.
I believe if you do this, all cover art you added will eventually be added back. At the rate it is currently adding, it may take a week, but it should all come back. I think it is scanning every music file and adding the image to the data base, which with 110,000 songs, it will be a long process. I have a Mackbook Air, Mid 2013 for reference.