The piston with slimeblock above can't launch a player in a 1 by 1 area. To create put a piston with slimeblock on top then surround the slimeblock including the player with obsidian or any immovable objects and update the sticky piston. The player wont be launched.
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Thank you for your report!
We're actually already tracking this issue under MCPE-62419 so I've requested this report be resolved and linked as a duplicate.
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