Bedrock not generating on the new blending border between old and new blending

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Oh, the world was created in 1.18 and now updated in 1.19 and I was exploring the nether roof to get to new chunks in the overworld, but here is a copy of the world I used:

I took a look at the level.dat, and found that WasModded equaled 1. If your world was ever loaded in a modded version (Optifine, Fabric, Forge, etc...) and the bug is world specific, it is invalid.

 Having the same issue in my world. The world was modded with lithium however so perhaps it's a modded issue:

(Modded -> Vanilla)
After just simply making a new world and upgrading it i can confirm its a vanilla bug very easy to recreate:

(Vanilla)

Reproduction steps:
Create a world in 1.18.2
Go to the nether, load some chunks
Quit minecraft
Play 1.19
Upgrade the world created in 1.18.2 to 1.19
Fly on the ceiling where new chunks will have generated

I had the issue as well and I was able to reproduce it (by following the steps Notoriousi already mentioned).

Link to the world: https://drive.google.com/file/d/1A8PVb6kfTrZzKyMcjArM9Qh6WtNEdoQZ/view?usp=sharing

I hope this helps.

Can confirm for 1.18.2 upgrading to 1.19.1 pre-1