I have done a test in the new pre-release versions of 1.18.
I create a world in 1.17 and update it to new pre-release versions of 1.18.
The blending feature works well.
Therefore, there should be three kinds of chunk, pure 1.17 chunks, blended chunks and pure 1.18 chunks.
Then, I used a tool called MCA Selector to delete chunks.
If I delete pure 1.18 chunks to regenerate the chunks, no problem exits.
However, if I delete pure 1.17 chunks or blended chunks, those chunks cannot regenerate well with blending feature
The game does not attempt to blend those chunks and the harsh chunk borders show up.

I hope that the blending feature can still work well if the chunks from 1.17 or being blending are deleted in 1.18.