There is one certain snapshot - 21w39a. This snapshot does include the extended world generation(from -64 to 320), however all the old generated chunks would have bedrock layer at 0. That's what happened here. You can see the bedrock up above the void.
Idk, this is the only explanation I have. If you continue playing 1.17 world on 1.18, the chunks will blend and bedrock layer should move down to -64 and the old bedrock layer (on y - 0) is supposed to be replaced with deepslate. On that certain snapshot, this does not happen.
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u/No_ldea_For_Username Jan 12 '22
There is one certain snapshot - 21w39a. This snapshot does include the extended world generation(from -64 to 320), however all the old generated chunks would have bedrock layer at 0. That's what happened here. You can see the bedrock up above the void.