A named zombie keeps disappearing in my testing world. This isn't the first time this has happened.

Once I tried to set up a villager transporter and used a named zombie to scare away delivered villagers. When I was a distance away, even when the chunks were loaded, I would return on the zombie was gone. He wasn't exposed to sunlight and whenever I was near the zombie nothing would happen.

Recently I've been keeping a name tagged zombie in a cauldron, and even when I was building in the very same chunk as the zombie, occasionally if I took my eye off him, I'd return to the cauldron to find that he had despawned.

It was my understanding that mobs should not despawn if name tagged, so I'm puzzled as to why this zombie keeps disappearing. I know he has not find away to kill himself as this has happened a handful of times now and rotten flesh has never been dropped.