Summary:
When the Caves and Cliffs toggle is enabled in an existing world, some chunks below Y=0 get deleted after reloading the world.

Steps to Reproduce:

1. Create a new world with the Caves and Cliffs toggle disabled
2. Leave the world
3. Enable the Caves and Cliffs toggle
4. Open the world
5. Dig below Y=0
6. Observe everything is normal
7. Reload the world

Observed Results:
Some chunks below Y=0 get deleted.

Expected Results:
Chunks below Y=0 should not get deleted.

Screenshot:

Notes:
This can also happen when returning to the Overworld via nether portal.