Was the phone dropped? Does they carry the phone in their front or rear pocket? Let me know the answers please.
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=== Update (05/10/2020) ===
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It sounds like the contacts on the outside of the board are losing contact or possibly have contacts pulled off of the motherboard. It has a split board and if you drop it hard enough or in the right spot. Or if you bend or twist the phone it caused this issue. This happens from putting the phone in your pocket. I have a iPhone xs max with a bow in it from someone who keeps the phone in their back pocket. I would watch a few videos on YouTube from jessa at ipad rehab ipadrehab.com or Jason from sts telecom ststelecom.com it will give you an idea. Look up videos for a iPhone X, XS, XS MAX they basically use the same board. If you send it to someone I would recommend one of them. I hope my information helps