The Bug:
Mineshafts can generate in the bedrock layer. Relates to Bedrock bug ticket MCPE-184894.
Steps to Reproduce:
Create a world with this seed: -7213800522284939450
Go to the following coordinates: 5891 -60 6044
You will find a mineshaft generated in the bedrock:
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Expected Results:
Mineshafts must not generate this near to the bedrock layer.
Observed Results:
Mineshafts can generate this near to the bedrock layer.
The mineshaft also seems to break bedrock.
Edit: Known instances of that happening is covered in MC-275037.