the problem is that the wither skellies dont spawn on dirt in a fortress and thus wither roses cant be used to filter out other mobs.

this works fine in main nether dimension but not in the custom one:

this is on a public server.

{
"type": "minecraft:the_nether",
"generator":
{
"type": "minecraft:flat",
"settings":
{
"layers": [

{ "height": 1, "block": "minecraft:air" }

],
"biome": "minecraft:nether_wastes",
"structures": {
"structures": {
"minecraft:fortress":

{"spacing": 32,"separation": 8 , "salt":xxxxx}

}
}

}

}

}

xxx is a number - i'm just blanking out the salt no