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