Every time I enter the nether and come back to the overworld, only a couple of chunks load in. Mobs still appear to spawn, but doesn't look like it has an AI, they only take damage once and becomes invincible. Reloading chunks and texture pack doesn't seem to fix the problem, but restarting the game can fix the problem, it's just a bit annoying having to restart the game after travelling dimensions. Also going to the nether can sometimes produce the bug, but only has happened to me once.