Worldgen is not generating the world as expected in 1.20 for a world created in 1.19

Partner Rank: 1
We have no feasible workaround for this for content already submitted.

Verification builds:
1.20.0
1.20.30
1.20.60

Impact:
This may have a major effect on reviews; players cannot play the map correctly.

The following Noxcrew content is affected by this bug:
Mermaid - e08aeecc-4ef5-4df6-993d-8bcae8c673dc
Unreleased content x 2

Notes:
We start with a world from bedrock generated with bedrock worldgen.
It was then converted to a Java 1.19.4 world and then converted back to bedrock 1.19.80 after we edited the world.
The seed is never altered so that the worldgen continues in bedrock.
When the world is back in bedrock, we change the following in the level.dat with NBT Studio to enable Survival world generation.

GameType: 0
Generator: 1

Repro Steps:
Download Mermaid Life from the Marketplace store (Minecraft version 1.20)
Start the map and enable cheats.
Teleport to 165 182 821

Observed Results:
The worldgen is not generating the world as expected.
(Left is in-game, and Right is the world generated before opening the world in the client)

Expected Results:
The worldgen is not generating the world as expected as in 1.19.81
(Left is in-game, and Right is the world generated before opening the world in the client)

Screenshots/Videos attached: Yes

Regression Build:
1.19.81