I upgraded a "Floating islands" type generator World to 21w44a and I went to the edge where the new chunks are and I saw that the world tried to blend.

How to reproduce
1. Create a Floating islands generator world in 1.17.1 or below
2. Generate some chunks in 1.17.1 or below
3. Save the world and load up 21w44a
4. Load up the Floating islands world you generated in 1.17.1 or below
5. X World blending is broken