Deepslate doesn’t replace stone below y=0

I’ve been using the recent experimental snapshots and have noticed that deepslate doesn’t replace stone below y=0. I know that deepslate will appear in blobs at y=16 to y=0, but once it’s below that, it’s only stone.

 

(The first image is below below zero, the second one is above zero)

I believe that this has been an issue before, but I thought it was fixed. If it can’t right now, I understand, it is an experiment.