mobs are randomly despawning

i lost 3 cats, 3 wolves, 2 villagers and 1 or more sheep and maybe trader llamas. i was 200 blocks away when my tamed cats despawned, 2000 when my tamed wolves despawned and 17000 away when my sheep and villagers despawned (not sure about the last one). also there was no death messages, the villagers were fenced and so was the sheep, and my difficulty was only on easy when i was like 1500 blocks away from my villagers.