Why can't I change album art?

Here's the issue.

I have 3423 songs that are in my library, all of which have messed up cover art. At one point I must have applied the same Jay-Z Blueprint cover art to all of my files on accident so now every song has the same Jay-Z cover.

I've made a smart playlist with the rule "Album Art = True."

I then went through all 3423 songs, deleting the cover art from the Artwork tab. I used a script/macro app so I didn't have to do this manually.

After running this script/macro, the smart playlist is empty, as expected.

After these changes, I left my iTunes open for another day to let the changes sync to the cloud.

I come back the next day and find that the same 3423 songs are back in the playlist, artwork still there.

I did this one more time. Next day, same results. The cloud seems to ignore my changes and keeps the art no matter what I do.

I read of a few elaborate workarounds that have you delete songs from your library and then re-add them but that's not an option for me. I have a great deal of playlists created that use many of these songs and as soon as I delete these songs from my library, those playlists are going to get messed up.

Please, Apple support, can you offer some assistance?