Question about the world height

I've tried allowing for a custom sculk block to generate below y=0, between y=0 to y=-64, with this new custom sculk to replace Sculk and only be placed within Deep Dark biomes, but it seems like generation doesn't work at all below y=0, I've tried even just making some custom block tests to try to generate natural blocks in the deep dark by replacing deepslate with just a test block, but nothing, but anything above y=0 is fine, and was just wondering if this was a bug or I'm doing something wrong.