Chunk blending doesn't work when loading old worlds updated to snapshot 21w41a

Old worlds without new world generations when updated to 21w41a cannot have it chunks updated when opened in 1.18.

Steps to reproduce:
1. Create a world in release 1.17.1 or any version below snapshot 21w37a

2. Update it to snapshot 21w41a

3. Update it again to release 1.18