I found this bug when I was making an iron golem in creative mode I haven't realised a few seconds after. What happened was that the iron block pieces you would place for arms was right next to or above the fern would stop the iron golem to not spawn after placing the carved pumpkin. I hope you see this page and fix it soon. P.S sorry I don't have any proof so please take my word for it.

Sincerely,

PrisonMars