The bug:
Worlds from snapshots 21w06a to 21w14a do not blend with the new terrain in 21w43a, resulting in sharp borders.
How to reproduce:
Create a new world in snapshot 21w14a
Load that world in snapshot 21w43a
Fly around until you
see a big stone wall at a chunk border
Observed results:
There are sharp borders
Expected results:
The borders should be blended.
This makes sense. Adding the blending technology to all snapshots would be a nightmare for performance, because each one has a different data version and there were major changes to world generation in a bunch of them.