It's repeatable too.
I can't make it. Can you make screenshots?
That printscreen does not prove it was a 0-attack or 1-attack creature before.heck, it was not even a mutant.
This is how it should work:Unupped regular creature->unupped regular skeletonUpped regular creature->upped regular skeletonUnupped mutant creature->unupped mutant skeletonNone of the screenshots given here so far contradict this.