This might not be a bug, but rather a feature though it is not noted on the Minecraft wiki.
This does not exist in the current version but does in the preview.
Double-tapping the "sneak" key toggles flying the same way double-tapping "jump" does.
MCPE-153094
Double-tapping "sneak" toggles flying
Resolved
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We're tracking this issue in MCPE-152973, so this ticket is being resolved and linked as a duplicate.
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