Mobs appearing dissapearing close to player

At least "passive" mobs are appearing and disappearing only a few blocks away from the player. 3-10 blocks maybe? Not sure exactly how far.I haven't played enough to see if it applies to all mobs. If one adds redstone devices, it's unplayable. it crashes all the time. I have created a different issue for the redstone already.
Thanks for all your hard work.
Mark