I was trying out biomes you'll go in the nether along with this mod, and I noticed that in the garden-like biome with grassy netherrack as the top block, gravel/stone/cobblestone/speleothems/flint/pebbles were generating in patches near cliffs or holes in the ground. I'm assuming it's a problem where the mod thinks this biome is an overworld biome and therefore generates overworld decoration.

The best way to go about fixing this would probably be to make a dimensional blacklist with the nether and end blacklisted by default (as this mod doesn't add anything that those dimensions make use of anyways). For now, I've simply disabled the biome that caused this issue to arise, and I haven't seen the issue since then.