The nether has a chunk error which takes any chunk and brings it down 64 blocks. So the roof is at y=64 and the floor is taken away from the void.
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This is most likely a bug try to restart Minecraft if that does not work it is most likely generation bug.
DuplicateΒ MCPE-140436.
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This is a bug, it happened to dallasmed65 in this video: https://www.youtube.com/watch?v=kCFwPiOPhN4Β at time stamp 6:12