The bug

Making the Spawn Size value for any ore/stone variant (e.g. diorite or gold) a value of 1, or 2 causes zero of that thing to appear in the world.

How to reproduce

Change all the Spawn Size values to 1 in the Customized GUI, create a world, and then investigate. A fill-replace of stone->air is a good way to have a quick look.
At a value of 3, it starts to appear in clumps of 1 or 2, and at higher values, it behaves more like one would expect.