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...
Environment
Java Version 21w16a Windows 10
works as intended. See https://www.minecraft.net/pl-pl/article/a-caves---cliffs-announcement