Upon reloading Minecraft after a crash there seems to be a random chance that any Mob type entity will disappear in a small quantity. Any kind of mob will disappear, but it seems to have a preference for Villagers. Even named mobs will disappear. The removal happens 100% of the time after a crash. An example would be having 10 villagers in a village before a crash, and upon reloading only having 4-6 villagers.