Creating a flat world and dig straight down to bedrock (but not actually break bedrock) will create invisible "Roof barriers" on top of you after you destroyed the 3rd and final layer of dirt, meaning you can't go up while you are in Survival or Creative. The only way to get through the invisible roof barrier is to either use Ender Pearls, Chorus Fruit, etc.
MCPE-43615
Flat worlds creates Invisible "Roof Barriers"
Resolved
Duplicate
0
Thank you for your report!
We're actually already tracking this issue at MCPE-43601, so we will resolve and link this ticket as a duplicate.
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