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.