Wither skellies not spawning on dirt in fortress in custom dimension

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