Passive mobs used for farming are totally despawning upon reload of a world.
I have a cow and a sheep farm. Every single session it starts only with only one or two (needless to say, if one, my farm is really ruined).
I tend to it, bring more, feed, reproduce, get the numbers to ~10
After a reload, back to one or two!
This is just game-breaking in survival!
Yes it really ruins the fun but it is a known, long-standing bug that hasn't been fixed yet: MCPE-8002