Mobs won't spawn on nether roof

I cannot get mobs to spawn on top of the nether. Went here for some easier ghast farming. I built some flat cobble and nether areas for them to spawn above and tried in the nether wastes and soul sand valley biomes. Not a thing will spawn and I afk'd for hours waiting.
steps to reproduce:

Get on nether

Build viable blocks for mobs to spawn

Observe no mobs spawn

Mobs should be spawning according to the internet
Note: I am in a multiplayer 1.16.5 server but I couldn't make this ticket unless I selected 1.17.