I was recently building an Iron farm but everytime I caught a zombie villager and made it hold an item and wear helmet, it eventually disappeared. There were 3 instances of the same happening, my observations of which are :
1. 1st Zombie was wearing a player given helmet and sitting in a boat. I never even left the loaded chunks but was busy with the farm. When I looked to the zombie after a interval of about 15 minutes, I saw the boat and zombie flesh. No Wolves/Goats/Golems are in the area.
2. 2nd zombie was wearing full chainmail armor, got him inside the zombie chamber of the farm. then golems proceeded to spawn in the Golem chamber indicating the farm was working when the zombie disappeared.
3. 3rd zombie was in a boat with stone in hand wearing iron helmet. despawned from the boat when I was less than 20 blocks away from him. Only remains in this case was the stone he was holding, no rotten flesh was found.
Comments 2
Thank you for your report!
We're tracking this issue in MC-3453, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki
on the first point you mentioned it could have been that the zombie was in the sun which makes the helmet take damage and after a while it breaks therefore the zombie starts taking fire damage and will die.