Mineshafts go through bedrock when generated at sub-bedrock levels.

I am not 100% sure what versions this affects, but it probably is more than just 20w20b.

What happened:

The floor of the mineshaft was at y=3 below where bedrock generates, causing it to go through bedrock.

What I expected:

It to not generate at those y levels.

There aren't really steps to reproduce here, just have to find a seed where this happens.