Upgrading a world from 21w06a-21w14a directly to 21w43a will not let new chunks blend with old ones. In my opinion, this is a bug because the game should try to upgrade any default world from any (fairly modern) version correctly. Since the game lets you open those worlds, it should also upgrade them correctly.

How to reproduce:

1. Launch 21w14a,
2. Create a new world.
3. Quit the game.
4. Launch 21w43a.
5. Upgrade the world.
6. Go to new chunks.

What I expected to happen was:

The new chunks would blend with the old chunks just like how they do when upgrading any other world.

What actually happened was:

What you see in the screenshot below.