We have a standard donut pigman farm above the nether ceiling which worked fine until 1.13.2-pre1. Now no pigmen spawn. After some testing, it appears that pigmen will not spawn when the z coordinate is positive. I built four platforms, one in each quadrant around 0,0, and no pigmen ever spawn on the platforms where z is positive. I have not yet tested this below the nether ceiling, but it seems likely that the same is true there.
EDIT: Affects neutral and hostile mobs in all dimensions
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Also affects the overworld. Spawning platform at 0,0 only spawned mobs in negative z coordinates at night (using seed 13 to get ocean at 0,0).
I'm seeing the same thing. Also seems to affect The End with much lower spawns of Endermen south of 0 (positive z). Included screenshots of a simple test in the overworld.
TiscanLP has made a video showing the (lack of) spawns:
Tested on a 129×129 spawning platform created at y=64 below the Nether ceiling centred at 0,0. Tested while standing at 0,0. Spawns of pigmen (and other mobs) were greatly reduced at positive z coordinates. Pigmen did spawn at positive z coordinates, but the spawns were rare. It was normal for no mobs at all to spawn in positive z coordinates.
The overworld needs to be tested.