The game ends when you try to destroy a stair block that is submerged in a water source.
Even if you break it with an ax or hand, the game will end in the same way.
Destruction of stair blocks that are not submerged in water is okay.
The game ends when you try to destroy materials that are submerged in water, such as stairs made of stone or even half blocks.
Thank you for your report!
We're tracking this issue in MCPE-128985, so this ticket is being resolved and linked as a duplicate.
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