I continuously have the issue of all wild mobs despawning while I'm still in shooting distance (well within my simulation distance of 12 chunks). I am trying to cure Villager Zombies, but when I catch one and go get my supplies nearby, I see them vanish before my eyes. It's ridiculous. Animals do the same thing. The only mobs that stick around are my pets (horses, Trader llamas, dogs, cats, skeleton horses). However, at some point, I had six llamas despawn and one skeleton horse despawn despite being leashed. 

It feels like the simulation distance is buggy and I wish it had more than 12 levels on Bedrock for PC considering that people like me can have great hardware than can handle it.