When you crouch and press the sprint key the game tries to make you swim an activate the elytra. This causes you to not be able to swim up very well but easily sink to the bottom. This also prevents you from swimming off of blocks.
Steps to reproduce:
Go underwater with an elytra
Crouch and walk forward
Hit the sprint key and keep holding W
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We're tracking this issue in MCPE-123946, so this ticket is being resolved and linked as a duplicate.
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