It still effects 15.4
Still affects build 3
Because of 14022 I suspected that this might be part of a more general bounds checking bug which would affect all mobs, but I had only seen the slimes spawn on the bottom slabs.
I know that top slabs don't prevent spawns those are all bottom slabs and the slimes still spawn. Please reopen.
They do spawn in the nether portals but can't escape to the overworld because they as teleported as soon as they spawn as mentioned in issue mcpe-14032
Ok I thought, based on the pc mob farm designs, that the intention was that mobs that entered from outside the portal would be teleported. But if this is how it is intended it it's fine.
They can't completely ignore bottom half slabs because they prevent the mods from spawning beneath the block they are on as can be seen from bug 14022.