In previous versions, no bedrock was exposed to air in the overworld because it was covered in blocks or lava. In 21w06a, the lava does not generate, exposing the bedrock, likely due to the lava level not being implemented in the adjusted world height.
Linked issues
Comments 2
Duplicate of MC-214781
I don't know if this is the right place to suggest things, but I recommend the lava get a random level like aquifer water levels. Perhaps it should be restricted to lower levels than the max water level, though.