So, I was playing the new snapshot, and decided to go to the lower levels to find some diamonds. The bug happened when I found a geode that was near bedrock level.
I mined down and noticed that the bedrock was replaced by the geode and that I could create an opening to the void.
Thank you for your report!
We're tracking this issue in MC-203867, so this ticket is being resolved and linked as a duplicate.
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