When a world created in versions 1.17.1 or older are updated with 21w43a, the biomes at the edge of the new terrain has not been blended even though the terrain has been blended.

This is most visible where the old terrain is a land biome and the new terrain is an ocean biome. The terrain will end up with large strips of various ocean biomes above the water level.

To reproduce:
1. Create a new world with seed 1 using an older version, 1.17.1 or older.
2. After entering the world, allow terrain to generate without moving and then exit.
3. Load this world using the test version (eg. 21w43a).
4. Head south from the world spawn until the ocean is reached.
5. Expected: Biomes of new chunks to blend smoothly with the old chunks along with the terrain. Actual: An abrupt transition to (new) ocean next to the (old) forest.