A large part of my collection has Front covers as well as CD/Medium art tagged already (provided by the fanart.tv plugin).

I just ran across an oddity when I opened one of my albums: While it had no metadata changes, it was showing the "Excellent match" state on tracks, indicating that something would be changing. It wasn't showing a new cover art either however, so this made little sense.
However, in the Cover Art Details I could see that the image for the Medium had changed:

Picard is not providing the "Keep original cover art" option though:

It seems there's no way to keep the art other then disabling the Cover Art Embedding entirely for these tracks.