If you hold jump next to a vine (not at feet, but at head level), it will enter fly mode, however when not next to a vine at all, you will have to double jump.
To reproduce: build 3 blocks up, on the bottom block, place another block.
Put a vine on the topmost block. Stand next to the vine and HOLD jump.
I have not tested this on preview/beta 1.19.0.32/33 yet, I will update later when I have the time to test it (if the bug still exists)
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We're tracking this issue in MCPE-154247, so this ticket is being resolved and linked as a duplicate.
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