There is no deep slate even though it is below Y 0

I'm using Minecraft BE 1.17 Caves and Cliffs Beta
I was curious about the deep slate, but at the Y coordinate of 0, the deep slate is created only slightly above, and the bottom is still a stone. So the deep slate will be less and the stone will continue to occupy under it. Obviously, even though deep slate is added, I think there is something wrong with generating only a little. We hope Mojang will fix this problem.