It seems problem is fixed in 1.6.8I also noticed that the behavior of the Album Art Viewer has changed, for example for the URL of the playlist items.
But for local files like mp3, the behavior hasn't changed. It is the same as mentioned here: Reply #5
When moving to the next file, the cover changes if the next file has a different album name or a different artist name, this is typical for "Front Cover".
However, this is not the case for "Back Cover" and seems to work as expected here.
I am attaching the video to demonstrate what was said.