Querz

This has been fixed with 1.21.5-rc2 with the fix for MC-295665

The game cancelling chunk blending after reloading a world appears to be a side-effect of this issue: MC-295665

As soon as a chunk gets updated from an older version to the world’s current version, it no longer blends into other chunks. The game only remembers that it updated that chunk in the current session, but “forgets“ that it updated that chunk as soon as the player quits.

In 25w08a and before, the tag blending_data had the purpose of forcing the game to blend a chunk with this tag into other chunks, especially between chunks with the same DataVersion. Since 25w09a the tag blending_data is added to the chunk when it is updated, but apparently has no longer an effect on blending chunks when using Optimize World or when quitting and reloading a world while chunks were actively blended.

It seems like chunk blending doesn’t work anymore between chunks with the same DataVersion. This is most evident when using “Optimize World“ on an older world.

Example:

1. Generate a world in 1.16.5.

2. Switch to 1.21.5-rc1 and use “Optimize World“ on this world.

3. Fly to where new chunks should appear.

4. Observe a hard edge in the terrain.

The last version where “Optimize World“ worked as intended was 25w08a. When repeating the steps above with 25w08a the blending works as expected:

I opened a similar issue a few days ago, assuming this only affects blending when “cancelling” the generation of surrounding chunks:

MC-295409