Blending does not work in the Nether & The End; old and new chunks create rough edges

The Bug

Blending does not work with Nether, the old and new chunks create rough edges.
It also affects The End, but it's not obvious.

How to Reproduce

1. Create a world in 1.15.2, 1.16.5 or 1.17.1. 1.15.2 is more obvious
2. Reaching the Nether.
3. Save the world.
4. Use 1.18-rc3 to upgrade the world.
5. Wandering around the Nether.
6. Until you find the rough edges of the old and new chunks.

Upgraded world download X:142 Y:70 Z:-289

Expected Results

Chunk blending should work in all dimensions.