Diamond Ore generates as Stone variant when touching Tuff below Y-0

From what I seen at least, Diamond Ore generates in its stone variant below Y-0 if its touching Tuff (If this is intentional, I apologize for the false bug). But realistically Stone doesn't belong below Y-0 and it should be generating in its Deepslate variant. Other Ores may be affected too, but I haven't seen it.