0.10.4, 0.10.5, 0.11.0, 0.11.1, 0.12.1, 0.12.2, 0.12.3, 0.13.0 Beta 1, 0.13.0 Beta 2, 0.13.0 Beta 3, 0.13.0 Beta 4, 0.13.0 Beta 5, 0.13.0, 0.14.0, 0.14.2, 0.14.3, 0.15.0 Beta build 1, 0.15.0, 0.15.1, 0.15.2, 0.15.3, 0.15.7, 0.15.10, 0.15.90.8, 0.16.0, 0.16.2, 188.8.131.52, 1.0.2, 1.0.3, 184.108.40.206, 220.127.116.11
Tablet - iOS - iPod Touch 5
Update by SuperGeniusZeb on 26/Apr/2017: This bug has been fixed in 18.104.22.168. If you are still having issues with entities apparently disappearing, see the separate issues
MCPE-1982 (Mobs and NPCs Can Escape Enclosures) and MCPE-21416 (Entities can despawn when entering/exiting the Nether (multiplayer)). Also, keep in mind that entities that have never been interacted with are SUPPOSED to despawn. When testing either of the aforementioned 2 bugs, please make sure to interact with the entities you're observing first. If the entity ever sets you as its target, follows you, or gets fed/bred/punched/etc. by you, that counts as an interaction.
First pic is of the cow-farm; every powered rail had a cow in a minecart...somehow two whole rows disappeared completely, without a trace of the minecarts or cows.
Next two are pictures of the underground villiage where despawning villiagers were supposed to go.
The rest after that would be some of the villiagers' holding cells where they are staying until the villiage is done and the villiagers are capable of door-opening and breeding and wont just stand right at a door while zombies smack them through it. You can see where some of the villiager cells clearly weren't wrecked by me releasing them, yet there is no villiager...and no way that a zombie could've reached it. There was only ever 19 villiagers, so it isn't like somehow a siege happened and zombies spawned right next to them.
Also, random sheep that are individually fenced off and within a larger structure, despawned randomly for no apparent reason as well. Same goes for another animal farm that was just a bunch of chickens and cows and pigs in a room underground.
Update by MissMarzenia on 03/Dec/2015
Thank you for all the comments. We reported this bug for further testing and hopefully it will be resolved in the future.
This is a VERY complex issue, we have been looking into for quite some time. It is not easy to reproduce and easily fix. It is also very inconsistent and therefore we are unable to fix it right away.
Just a reminder - We are currently using internal tool to test and assign bugs, so we do not assign bugs to developers in JIRA, it doesn't mean we are ignoring them.