Phone powers itself on but not off

Unusual one,

An iphone 6 plus i have replaced the screen on is powering itself on randomly after being turned off. It doesn’t turn itself off though when it’s on. Quite confused! I’m going to try a new battery and possibly the power button flex after that.

Has anyone had this before?

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Does it seem to charge fine? It’s posible there’s some debris in the charge port so it’s falsely “detecting” it’s plugged in when it’s not.

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