When blending old chunks, some caves are filled with water where they shouldn't

How to reproduce:

1. Create a world in version 1.21.3 with the seed: 7398416967226801501. Make sure the render distance is 8 chunks.

2. Teleport to x: 130 y: 100 z: -180 and let the world load without leaving the chunk you are in.

3. Update the world and fly straight west for 12 chunks.

Expected result: the cave that was half generated in 1.21.3 will have blended without filling it with water.

Actual result: The newly generated chunk of the cave is filled with water that stops on the chunk border. When looking underground you can see that there is also water placed there (see attachment).