Mobs which are intended to be persistant will occasionally vanish from the game.
I have created a village in which there are trapped persistant mobs; villagers, tame cats, and tame dogs. Periodically, if the chunk containing said mobs is unloaded and then reloaded, one or more of them will be gone. I have performed tests and ensured they could not have escaped or have been killed (fully enclosed, peaceful mode). This happens in both single player and multiplayer. I suspect they may be falling though the floors, but have been unable to verify it because if the mob is loading before the chunk, it would fall into the void. It is tricky to duplicate, as it seems to happen at random. All of the mobs may remain through many chunk unloads and loads before one will suddenly disappear.
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I put villagers in holding cells inside the village itself so they wouldn't wander around for trading purposes, this is the second time, all of them have disappeared. first time i thought they were too far outside the village boundaries, so i destroyed those holding cells and put them all inside the village boundaries. I went to the end, got killed. and when i got back to the village, all the holding cells were empty again. it took me awhile to get all those villagers in those cells, and have them as suitable trading partners, now they are gone. this needs to be fixed.
Duplicate of MC-1057 and MC-1407, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.