Blending does not work with worlds from old 1.18 snapshots

The bug:

Worlds from snapshots 21w06a to 21w14a do not blend with the new terrain in 21w43a, resulting in sharp borders.

How to reproduce:

1. Create a new world in snapshot 21w14a
2. Load that world in snapshot 21w43a
3. Fly around until you
4. see a big stone wall at a chunk border

Observed results:

There are sharp borders

Expected results:

The borders should be blended.