Summary:

Dirt can generate below y=0 from mineshafts. This bug was fixed in Java MC-215876, but not in bedrock.

Generation example:

Seed: 123
Coords: 1170 -36 985
Coords: -90 -32 244

Steps to reproduce:

1. Create a world with Cave and Cliffs (with the specified seed).
2. Visit the specified coordinates.

Observed results:

Dirt room generated below 0.

Expected results:

Dirts below 0 should not be (including dirt rooms).

Image: