To make a Nether thats 256 blocks tall and to get around MC-187716, many people change the surface builder to the Default type in Nether biomes so that the surface builders generate above 128y. However, when using default type surface generators to fix the issue, I found that in 1.17 default type surface generators dont generate anything under 50y. I haven't tested this bug much, but it doesn't generate either the  top_material or the underwater_material. In the datapack I linked as an example, both Crimson and Warped forests have a Default type surface builder, and in both of them their nylium does not generate at or below 50y.

The surface builder code for the warped forest is: 

{{{}}
  "config": {
    "top_material": {
      "Name": "minecraft:crimson_nylium"
    },
    "under_material": {
      "Name": "minecraft:netherrack"
    },
    "underwater_material": {
      "Name": "minecraft:crimson_nylium"
    }
  },
  "type": "minecraft:default"
}