Zombie pigman dont spawn in a very high level on nether

I have made a gold farm in the block Y:253 and the zombie pigman dont spawn there . Ive already tried leaving the game closing minecraft copying the world but the pigman dont spawn there.

sometimes 2 or 3 pigman spawn but before I kill them no more pigman spawn.

I think its a bug because in 1.8 the 80 pigman spawn normally.

Thanks.