When breaking water with a piston, it leaves behind a ghost block of the water that has the same properties as normal water, but cannot flow. The Ghost block goes away when in contact with water or interacted with.
The video shows how to recreate the bug.
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I thought this might be due to an unofficial copy of the game, but a Switch user has made this video that shows it in the second half. https://youtu.be/P_PqBf9p6Uw
I still cannot reproduce it with just a piston, but I can reproduce it by pushing a stair into the water and then blowing up the stair with tnt.
Tracking this now at MCPE-178142.
I cannot reproduce this issue. Where did you obtain your copy of the game?