Mobs such as horses and villagers seem to despawn almost at random usually in small groups of about five at a time this doesn't happen all the time but I regularly return to find half of my villagers gone (they are in an enclosed space on peaceful mode)

How to reproduce: I'm not entirely sure what causes it but my guess is that it occurs either upon unloading and reloading the mobs, or upon closing and reopening the world, it also may have something to do with me leaving my world open for long periods of time.

this issue or similar issues has been put on the bug tracker before however they are marked as "resolved" which is simply untrue.