I built a large, resetable iron golem farm. One of the villagers in the farm consistently disappears. I placed it in its holding cell once, eventually discovered it had disappeared, assumed it had been struck by lightning, turned into a witch, and despawned, and built a protective covering to keep lightning form reaching the villager. I later replaced the villager and fixed some other issues with the farm, and the farm ran fine. I've logged off and back on once since then. The replacement villager in the same location has also disappeared. Relogging doesn't cause it to return, it's really gone. I'm in a survival single player world on normal difficulty. The villager is in a 1x1x2 space. The farm is in the spawn chunks and above y=200. The villager is in a satellite container, a substantial distance away from the rest of the farm (but always in view), and is used in the rebuilding process. It's standing on top of a solid block which is on a sticky piston that moves the villager up and down.