Summary:

Mineshaft can be generated in bedrock. In this case, the mineshaft does not replace the bedrock, but is simply generated in the bedrock. The bedrock can block the entire mineshaft corridor and it doesn't make any sense.
Also very often the mineshaft is generated so low that the bedrock is visible on the floor. I believe that the bedrock should not be on the surface.

Examples:

Seed: 1792249514
Coordinates: 2330 -60 1076
Coordinates: 2368.55 -59 1083.75
Coordinates: 2370 -61 1110
Coordinates: 3533 -60 1107
Coordinates: 4603 -60 4858

Steps to reproduce:

1. Create a world with the specified seed.
2. Visit the specified coordinates.
3. Consider generating mineshaft.

Observed results:

Mineshaft can be generated directly in the bedrock (not replacing the bedrock).

Expected results:

Mineshaft will not generate in bedrock (so low).

Image: