16.3 ported my save from previous version. (quest accepted before version change if that makes any difference)
Went to the tunnels to complete mouse prince's knights inc. quest, came across a mouse in mech who wanted a quickie, after that every hourly sister I could talk to (first time going in the kingdom, 2 outside to the north) had the naked mech mouse sprite, after talking to the queen the normal sprites came back for the mice outside to the north.
An Imgur album with screenshots and small descriptions too.
Did some more testing while I was writing this:
Having fun time in the city with a Lamia or a mouse (haven't tested with other but I guess it would work with every makes the avatars go back to normal for hourly sisters.
Walking out of clocktown from south resets the avatar.
Saving in town and loading the save doens't fix the avatars.
Talking with the mouse shop on the left side of the town fixes the avatars (mouse shop person has her own avatar).
Did something other too that fixed the avatars back while running around in town but can't say what it was.
Edit: Thought I'd mention that nothing seems to break so much that one wouldn't be able to continue playing, just the avatars getting mixed up and then fixing itself back