Summary:

This glitch happens when using the Caves and Cliffs toggle on a 1.17.30.24 beta world. When you enter a nether portal and explore the nether and then come back to previously explored chunks, when the chunk is reloaded the whole chunk gets shifted down 64 blocks with the blocks that go under 0 on the y-axis just completely dissappearing.

How To Reproduce:

1. Create a world on the beta version 1.17.30.24 with the Cave and Cliffs experimental option toggled on
2. Go to the nether through a nether portal
3. Once spawned in, travel about a 400-500 hundred blocks away from the portal, enough that already explored chunk are unloaded
4. Come back to the previously loaded chunks so they can reload and then the problem will reveal itself

Observed Results:

Chunks that reload are shifted 64 blocks down on the y-axis with the lower half of the chunk that would theoretically reload in from 0 to -64 not loading in at all.

Expected Results:

Chunks are fully loaded between 0-128 with no parts of the chunk cut off.