Mineshafts can override bedrock blocks

The Bug:

Mineshafts can override bedrock blocks. This is not consistent with Bedrock Edition, where mineshafts generating in the bedrock layer do not override bedrock blocks. This strongly relates to MC-275027, but does not duplicate this report. MC-275027 will be discovered when testing this bug.

Steps to Reproduce:

1. Create a new world with the seed: -7213800522284939450
2. Drink a potion of night vision.
3. Go to the following position: -8313 -62 -7795

Expected Results:

Mineshafts will not override bedrock blocks if they happen to generate at the bedrock layer, just like what happens in Minecraft: Bedrock Edition.

Observed Result:

Mineshafts will override bedrock blocks if they happen to generate at the bedrock layer, generating in place of where bedrock blocks should generate.

Note:

This does not duplicate MC-275027, because this report is talking about mineshafts overriding bedrock blocks. Mineshafts being allowed to generate at the bedrock layer is covered in MC-275027.