When loading a world created in 1.16.5 (and later updated to 21w40a dirrectly) in snapshot 21w43a , chunks generated in 1.16.5 still have the bedrock layer and no deepslate beneath it and a deepslate wall between chunks generated in this snapshot.
Reproduce:
1) Open minecraft in snapshot 21w43a
2) Open a world created in 1.17.1 or bellow with chunks generated in those version
3) Check layers bellow y0 and the borders between chunks generated in this update
Invalid. You stated you loaded the world in 21w40a, which does not support chunk blending, but still changes the world to a 1.18 world. In return, a 1.18 world doesn't need to blend with a 1.18 world.