I've been trying to troubleshoot this for a couple of weeks now, however I am unable to solve it.

I have created an iron farm, in which a zombie is in a mine cart on a square track and it goes around scaring villagers intro producing an iron golem.

The area that they path around is covered, they are named, they have an item in their hand, they are also wearing helmets, and still they despawn 100% of the time. Sometimes it's as soon as I leave the chunk, on this latest round I logged off for an hour, came back, played for another 30 minutes and then it disappeared.

I have moved the farm so that the entire track is within one chunk, I have made sure that the zombie was spawned within the same chunk as best I can, they are still despawning.