I started a survival world on 1.8.1, and locked the difficulty to "hard" on the first day. This changes and sometimes it unlock itself when entering the nether and so, but there is something strange related to spawnings, and I think it is related to the difficulty locked.
Usually mobs dont spawn where they should, even if staying 23 to 32 blocks away (tried every possible block, 1 block per night afk on a slime farm) and sometimes mobs dont spawn... I dont really know why and I havent seen any other report like this.
This doesnt happen always. Maybe a night I go to explore around and I see TONS of mobs, but then the next night I go and there are no mobs, or 3 creepers... I have a with hut and the area on 148 blocks wide is all watered, and caves lightened, but witches sometimes dont spawn.. even staying 23-24-25 blocks away from the spawning area.