This is because of a block tag, if a ore generates next to tuff, which now generates below y=0, it is the normal stone ore texture. This is probably one of the things Mojang will be working on in the coming weeks.
No, if you read the article about the snapshot it says that there should be no dirt in negative y coordinates as well as granite and diorite, and the image shows that I am at y -35. The article is: https://www.minecraft.net/en-us/article/minecraft-snapshot-21w07a,
Also, the dirt is clearly not the starting room of the mineshaft
Also as a coal deepslate block would be hard to discern it may need to be replaced with a coal block