Pigmens does not spawn in nether portals anymore

When i was building an overworld goldfarm in the snapshot 18w50a, i discovered a bug. When i activated the nether portals no pigmans did spawn.i watched it for like 30 min, but still nothing happens. After that i coppied the farm with an structure block in an world in 1.13.2, where the farm worked without problems.
I hope that it is not on purpuse and that i could help find this bug to improve Minecraft