Disappearing iTunes Cover Art Updates

I have an iTunes Library on my iMac with over 1K albums and 15K songs. I spent several hours manually updating old lower-res cover art across several hundred albums to higher-res versions. I have iTunes Match enabled, and assumed that over time my updated cover art on iTunes would make it's way to the cloud and back down to my other devices.

Twice now, after shutting down my iMac and within a few minutes of restarting iTunes (not immediately), iTunes went into a several minute long unresponsive bouncing ball situation while my hard disk has lots of activity. I did not see that Genius or Match were automatically updating. During that time, the only action available with iTunes was to Force Quit, which I didn't do. iTunes eventually comes back, but when it does, all changes to the cover art are gone. I ran disk utility to verify I have no problems on the disk. Last night when I started to update the cover art again, I even shut down iTunes after every few dozen album updates, restarted it, and randomly verified the changes I had just made were there. Unfortunately, this morning on power-up, the unresponsive situation with iTunes occurred again and all cover art changes are gone. I have checked random songs where I updated cover art and verified that only the single original low res version of the cover art is in the "Artwork" tab on "Get Info" for the particular song. All artwork updates I made have vanished. I made one set of changes last night to an album not related to album art -- those changes are still there, so it confirms my whole iTunes Library has not regressed in some way.

Unless this is a pretty severe iTunes 10.7 issue, it would sure seem like iTunes Match is doing one of it's background syncs and "downgrading" my cover art back onto my iMac to the original versions up in the cloud.

Has anyone else had problems with disappearing cover art updates?

...or can anyone confirm if iTunes Match will accept "replacement" cover art as I would hope it should?

Thanks for your help!