Below zero height should actually generate Deepslate, but in the latest Bedrock Experimental snapshot (i.e. the kind of snapshot without the Xbox Insider Hub) does generate Stone.
MCPE-135230
Below height 0 generates stone instead of deepslate.
Resolved
Duplicate
0
Vinzent Brenner
(Unassigned)
Unconfirmed
Windows
1.17.10
Retrieved
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We're tracking this issue in MCPE-125117, so this ticket is being resolved and linked as a duplicate.
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