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I've come across this issue 3 times in the last week, and there's one point in common with all 3 devices : missing small components next to the battery connector on the motherboard (see image). I've done research, and this seems to be the problem. I can't tell you exactly what they do, but it seems related to charging the battery/not detecting the battery. If you don't have a battery plugged in, the phone will boot loops, same as this issue. I've put a new battery in that was charged, and all 3 phones turned on, but wouldn't charge even though the charging icon was present. The battery depleted until the phone turned off, boot looping again.
I've sent the devices to someone that can do micro-soldering, and will update when I get the result.