Entity render distance in bedrock is extremely low.  Mobs spawn in and out of view while moving since you can only see them as far as about 4 chunks away.  Kindly note that this has nothing to do with render distance (View distance) or simulation distance (Distance in which crops grow, etc).  This is the limitation on how far away you are able to see "mobs" like sheep, for instance.  It makes no sense killing all cows around your home, walking away for 10 seconds, turning back and cows spawn again.  This is also a huge issue in the nether, where you need to consider multiple hostile mobs, especially Ghasts that attack from a distance.  Please fix this and please understand how important this is.  The game feels unplayable.