I have noticed that everytime a iron golem dies/despawns/disapears, the villagers do as well. A couple of friends and i experimented with this in several different villages. The work stations for each villager was intact and not touched or moved. Each villager was either never attacked or was never outside during the time they despawned or disapeared. We even spawned several golems to see what would happen and they stayed until one of us logged off then it seemed like one after the other they would disappear. 

Hopefully this review helps and can be explained in further detail later.