I'm not sure if this is intended or not, but it's really annoying. When a mob is set to be persistent using NBT tags. It will stand absolutely still. It won't wander around like it normally would, instead, it just stands there and will look around aimlessly. This only happens with mobs such as blazes, skeletons, zombies, and even squids. These mobs will still move though: silverfish, slimes/ magma cubes, and bats.