The bug

Starting with 21w06a, noise caves can generate at any height they want, which causes some strange effects. For example, the mountains are all broken up now because a bunch of noise caves just cut right through them. Normal caves fail to generate if they are exposed completely and have almost no walls. But these new ones (and sometimes geodes too) can generate however they want, making weird random holes everywhere.

How to reproduce

Seed: -8610962999447939582
Coordinates 1: /execute in minecraft:overworld run tp @s -206.91 72.00 -103.86 -43.20 39.00
Coordinates 2: /execute in minecraft:overworld run tp @s 206.03 71.00 -351.30 242.55 -15.30