Ghast spawning seems to require a smaller area for ghasts to be spawned in the world. They should require 5x4x5, and when playing things like slabs and end rods every 5th block (4 air blocks solid block under this) then a slab/end rod). This pattern repeated over a good area, should block spawns but it doesn't and ghasts keep spawning (affecting my nether hub).
I have even placed more down to make a smaller area and they still spawn, so I can only assume the currently need a 1x4x1 area to spawn, which can be really bad for nether builds.
Hope this can be fixed