Mob Models Standing Still on Realms But Still Moving Collision

It's kind of hard to explain, but I'll try anyway.

My friend has a 10 player realm that we play on frequently. Sometimes, all of the mobs in a certain radius stop moving the models and look as if they're standing still. But their collision is still moving around. For instance, a creeper can look like it's standing still in one place, but then it blows up right next to you, far away from where the model is. This is a recurring problem that we seem to keep facing.