Do you put your phone in your back pocket? I’m wondering if it’s causing the layers of the boards to separate due to flexing when pressure is applied.
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EDIT* Now that I think of it, I wonder if it’s an issue with the charge port flex. I once replaced the charge port on a X and about a month later the customer came back with an issue where it would shut off if you put too much pressure towards the top right of the phone. I opened it up and was stumped for a few minutes. But upon inspecting the screen brackets I noticed one was bent. It was cutting into the charge port flex and when you applied pressure it would cause a short and restart the phone. I bend the bracket back and replaced the port and he hasn’t had issues since then.