In Minecraft snapshot 21w16a, newly created worlds does not have anything spawn below y=0 and has bedrock blocks at y=1, tested multiple times on multiple worlds and all have the same problem despite still spawning copper ore and stalagmites etc...
MC-224273
Snapshot 21w16a doesnt create below Y=0
Resolved
Works As Intended
0
works as intended. See https://www.minecraft.net/pl-pl/article/a-caves---cliffs-announcement