Passive mobs DESPAWN upon game reload

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!