Dirt can generate below y=0 from mineshafts

The bug

In the 21w07a snapshot, dirt is said to stop generating below y 0, but mineshafts still generate dirt rooms as part of their structure below y 0.

How to reproduce

Seed: 1717221929747746810
Coords: /tp @s -612.30 -49.09 -113.43 -574.20 36.15