When you load a superflat world from 1.17 or earlier, they don't get moved down to -64 so there is empty space below bedrock. I think this is a bug because if you create a new superflat world with the same settings, the bedrock is at -64.
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Actually, imo I think this could be a great feature, you could make an under bedrock map or even a basement for your existing superflat house!
I think it's intentional as moving the Super flat down can cause problems with custom maps.