It might be a problem with the rear cover, since the wireless charging pad ,the flashlight and a chip that helps the iPhone identify whether the rear cover is "genuine" is connected to the rear cover flex cable. When I was replacing the rear cover on my 15, the same problem occurs when I booted it without the rear cover installed. Also, there has also been cases of iPhone 15s restarting randomly with a defective rear cover flex cable. You should probably check that cable and the press connector to see if there's anything wrong with it.
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It might be a problem with the rear cover, since the wireless charging pad ,the flashlight and a chip that helps the iPhone identify whether the rear cover is "genuine" is connected to the rear cover flex cable. When I was replacing the rear cover on my 15, the same problem occurs when I booted it without installing the rear cover. There has also been cases of iPhone 15s restarting randomly with a defective rear cover flex cable. You should probably check that cable and the press connector to see if there's anything wrong with it. There might be problems on the charging port as well, so I would suggest you to get a rear cover and a charging flex from a working 15 and also check the motherboard for any damage, since there could be disconnected solder points between the two layers of the motherboard that's causing the problem.
It might be a problem with the rear cover, since the wireless charging pad ,the flashlight and a chip that helps the iPhone identify whether the rear cover is "genuine" is connected to the rear cover flex cable. When I was replacing the rear cover on my 15, the same problem occurs when I booted it without the rear cover installed. Also, there has also been cases of iPhone 15s restarting randomly with a defective rear cover flex cable. You should probably check that cable and the press connector to see if there's anything wrong with it.