I created a large stone spawning platform at y=253 in the nether. Pigmen will spawn when i arrive at the platform, and then no pigmen will spawn while i stand on it, despite it being 32x32.

I downloaded the world and played it in singleplayer (java edition) and had no issues with pigmen spawning on the platform while I am there.

I am the only person on the realm, and there are low entity counts and no close (<128) spawnable areas.

Maybe it's a render distance thing? Here are screenshots from the realm and from the downloaded identical singleplayer world.