I made ilmango’s mob farm tower in a self created void world. I had to make a void world due to the 128 block vertical rule being ignored. I created 9 spawning platforms with the top two being slabs and the bottom seven being cobblestone. Mobs never spawn on the top two layers. I even flooded the bottom seven and I might get one spawn in several minutes.

https://bugs.mojang.com/browse/MCPE-28587 this report might also be affected by slabs although overall spawn rates do not seem as prolific as on java edition.