I managed to reproduce it, here's how it goes:
When I replace an image under skin attachment,
in the source Images folder - the new image goes green, the replaced goes red.
When I duplicate the skeleton,
do the same process,
in the source Images folder - the new image goes green, but the replaced does not go red. It stays gree. It should go free.
When I save and reopen it - it behaves as it should - used images are free, not used are red.
Here’s a video.