I got very confused and put a custom skin on the wrong pet. (This isn't the bug.)  I assumed that the skin would go on my active pet, but instead it looks like it went on the pet that had been active when I loaded the list of skins. 

The possible but is that the pet that I put it on was questing at the time. It says on the list of custom skins that pets must be active to add custom skins, and specifically says that they cannot be questing. So I thought that this might be a bug.  

Pet in question: https://beastkeeper.com/pet/1363992 

If this isn't a bug, sorry to bother you with it. 

Thank you for reporting! Yes, this looks like a bug, and I can easily reproduce it. We'll hop on it after finishing all Tundra preparations.

I added a Rainbow Jar and a Gray Jar to your inventory to compensate the wrong skin application. Sorry for the inconvenience!

Thank you so much for the prompt reply & for the jars!