The bug

I upgraded a very old world to this snapshot. There are very weird sand patches at the border. The biome at the sand is beach, and at the grass is ocean
At the same coordinates in a world with the same seed there is just ocean, so that is not the problem

It sometimes also happens with 1.17 worlds

How to reproduce

1. Make some worlds in 1.17 and load all chunks within a a specific render distance (don't move)
2. Update to the newest pre-release and find the border
3. Go along the border and you will sometimes find this

Details of upgraded 1.17.1 world

World Type: "Default"
No additional generation settings
Seed: 9184991237633477144
Coordinates: 118.05 135.51 -293.96
Render distance used: 16
Upgraded World download: https://drive.google.com/file/d/1zcHNn11jWrN8UzUpUWHfEiAXWnoPVl07/view?usp=sharing

World download of world 1 (upgraded infdev world): https://drive.google.com/file/d/1OcUQcba5s45Kt6-bl_0vq5Qo4VK6BA0I/view?usp=sharing