Well it did. I 've just checked the sync between my media drive and backup drive and 380 Folder.jpg files were over-written as a result of the library import (that's probably all the ones for which there was cover art available on the Internet).
It never used to do this.
The only thing that's slightly unusual about my set up - my Media drive is ext3 format and accessed via a SMB share. Could it be that MC is "failing" to see that Folder.jpg exists because of this and some recent change?
[EDIT]
Think I've found what's going on and why I didn't used to have this issue (and how to avoid it)...
As above, my media drive is accessed via a SMB share, which I have mapped to A: in Windows. This gives me two ways of specifying what folders are added to the library in the 'Add Folder' window in MC - one is it to pick them from the Computer tree (i.e. picking the mapped drive A:) the other is by via the Network tree.
I had been specifying the folders via the mapped drive, but after a OS rebuild and starting my Library rom fresh, I thought why not go via the Network tree.
When I do that MC finds all my audio files first time, but it doesn’t find my Folder.jpg files. It then either overwrites what’s there by downloading the cover art from the Internet (which under certain conditions (that I haven’t been able to narrow down) it does regardless of whether ‘get cover art’ is enabled or not…) or it doesn’t display anything.
If I add the folders via the mapped drive, all is well – it fins my audio files and cover art files and so there are no issues (I think…)
Anyway, regardless of the advice above, that’s what I’m seeing. This suggests that there are three bugs – one with adding folders via the Network tree, one with the ‘get cover art’ option being invoked erroneously, and one with a not terribly robust check on whether Folder.jpg exists before MC decides to download cover art.