The Amazing Transforming Wolpertinger! [Bug?]

Not sure if this is intended or not, but my wonderful little Wolpertinger cast a spell that stole the innate ability of an Iron Golem. Curious, I attempted to extract the Iron Golem’s core with my Wolpertinger and, wouldn’t you know it, he turned into a Nightmare Golem. And stayed that way after battle. During the battle, he had a nature symbol, but how he’s turned back to sorcery.

Never trust a Wolpertinger.