To test this first unplug battery. Plug battery back in. Don't press power button. Plug charging cable in and see if iPhone turns on itself. If not then chip is most likely bad.
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And yes this is caused by the user of the iPhone using cheap chargers or car chargers with cheap iPhone cables that are not MFi certified.
Could be U2 IC charging chip gone bad.
To test this first unplug battery. Plug battery back in. Don't press power button. Plug charging cable in and see if iPhone turns on itself. If not then chip is most likely bad.