This might be one bug, or it might be two. I'm reporting this for the server because I do not know exactly which component the bug(s) are in.
First, I've noticed that occasionally when uploading multiple cover art files, MB will occasionally drop the "type" tag from the file in the first slot. I usually place the "front" artwork in this place, so this manifests as the cover art uploading without type, and either the back artwork or the amazon artwork showing in Picard. This could be chalked up to carelessness, but it's happened too many times even when being extra careful to ensure that 'front' is properly checked before submitting the edit.
A release where this has occurred is here: http://musicbrainz.org/release/7c5bad7c-1b9d-4621-8d14-ec01898e15c6, and the specific edit record is here:http://musicbrainz.org/edit/31642780
Second, for releases where this has manifested, subsequent edits to correct the type of image (example: http://musicbrainz.org/edit/31826028) result in the affected 'front' image displaying correctly in MB but when downloading in Picard the image type remains 'Unknown' - Picard continues to show the back or amazon artwork instead of the artwork set to 'front' in MB.
Configuring Picard to download all image types results in three images using "Info" (screen shot attached): The cover artwork (shown as Front in MB) is listed as 'Unknown" in Picard, the amazon artwork listed as 'front" in Picard, and the back artwork as 'back' in Picard.
I was able to correct this in another release by removing the affected artwork and re-uploading. The issue was corrected after the edit period expired.