Ores, Gravel, Dirt, and Stone Variants do not generate below y= -4

Ores, Dirt, Gravel, Andesite, Diorite, and Granite only generate down to y= -4 and do not generate any lower.

To reproduce:

1. Generate a new world with "Caves and Cliffs Features" enabled
2. Run the following command to explore down to y= - 64:
/fill ~-1 -63 ~-1 ~1 ~ ~1 air
3. Observe that no solid blocks generate besides Stone.

Admittedly, this may be a place-holder rather than a bug, because the Deepslate generation features haven't been added yet.