So I was playing in my world, and had a bunch of foxes and a horse tied to fences. I went to nether portal and my mobs weren't there when I came back, so I thought it must be MCPE-21416 .
After a while I attached couple of goats near the same place temporarily and after some time the goats weren't there too. I didn't go to nether this time and multiplayer toggle was turned off. Here are the fences from where mobs disappeared and chunk boundaries are marked with redstone dust.
This issue is covered in MCPE-21416.