Chunks can corrupt and reset to default world generation. I'm pretty sure any chunks loaded pre Caves and Cliffs update will reset to what it would be post update.
If I'm not mistaken it has something to do with running out of RAM or Memory on your device which triggers the bug.
Below is some screenshots/examples of this glitch
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[Mod] EVGENSYPERPRO
Thank you for your report!
We're tracking this issue in MCPE-150757, so this ticket is being resolved and linked as a duplicate.
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