Seemingly at random, some chunks will have all blocks below y = 0 removed, while others are unaffected. Idk if this helps but all of these chunks were rendered pre-1.17.
In the picture, the tunnel both at the bottom and the top of the pic were fully completed.
Thank you for your report!
We're tracking this issue in MCPE-156679, so this ticket is being resolved and linked as a duplicate.
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