Me and my friend are unable to obtain 'Iron Belly' either, we have tried on 3 different worlds, in all difficulties except peaceful, all made in survival, and none have been in creative. We have tried multiple ways to burn hunger, and eat rotten flesh at different health/hunger states, but have had no luck. This is the second to last trophy we both need to get. I hope there's a quick fix for this.
Gosh, the timing. Just when I want to trophy run, they throw out an update that breaks this.