On multiple occasions I have loaded my world to find that any farm that I have, (whether cows, pigs, sheep, or horses so far), and most recently a village I created, have all disappeared. Friendly mobs will still spawn normally, but if I create a farm, they don't seem to load and I have to start all over again. I can't say what is triggering this, except it only occurs when loading a saved world. The world is in Survival, with normal difficulty. I can only suspect it's after a certain threshold, as I do recall my sheep didn't disappear until I had around 20 bred, along with the cows. I had 13 villagers (one for each workstation) and they only disappeared when the number was increased to 20 to begin spawning Iron Golems.