Unless the world its completely generated I walk further unless an invisible wall in my land and mobs in the invisible wall they don't move they shake. And when I leave world and go back in my world the invisible wall dissappear and mobs move the correct position. It happens all betas, its not a barrier I hold the barrier in creative nothing shows up. Try mojang to figure it out if nobody believes me use your Android device that has a beta and play it and walk further.look