Passive mobs despawning (even with nametags!)

When my MCPE crashed or I leave my game, passive mobs like (skeleton) horses, villagers, iron golems, pigs, sheep, cows, rabbits, mooshrooms, and chickens that I have kept in a generated chunk (viewable or non-viewable chunks) despawned even I have nametags on some of them.

Steps to reproduce:
1. Make your game crash or leave your world
2. Re-join the world that you have a lot of passive mobs
3. Check whether they despawned or not, this bug doesn't always happen