Hostile Mobs do not spawn 1.16.1

Created a pigman farm above the nether on a Minecraft Realm. However no Pigman spawn. 

Tested the exact same farm, at the same build height on a Single player world. Works fine. 

I have my spawning layers at y= 236 & 239. The spawning layers are magma blocks. 
The spawn layers are 19 blocks away from my afk spot

I have tried changing my render distance, logging out/logging back in. I have spawned proof 200 blocks in every direction. Mobs spawn in the overworld and nether, just not the farm. 
It doesn't make a difference how many players are online or in what world.

Spent a good few hours collecting the resources and building just to find out Realms has a bug. Not very happy! 

Difficulty: Hard 

Render Distance: 10 Chunks

Version: 1.16.1

 

This needs resolving as it works on every minecraft server other than Realms. Will not be paying for another month and will not be recommending it.