In vanilla Minecraft, the reason that mobs do not spawn on top of the nether is that mobs do not spawn on bedrock. The stacked dimensions world option however replaces this bedrock with obsidian, thus ghasts will spawn on top of the nether and make really annoying sounds, with the player not even being able to reach them due to the ceiling portal thing.

Another solution is to generate one more bedrock layer on top

I made it compatible with netherHigher but unfortunately even if the nether is 256 high mobs can still spawn on top

To solve this I am going to make it compatible with https://www.curseforge.com/minecraft/mc-mods/netherhigher

fixed