I'm having the same problem with an old iphone 6s plus, which previously had a black screen, unable to turn on, so I decided to send it to repair last week after sitting inside my drawer for two years, took them 3 days to fix the freaking screen, they told me that the phone backlight was out , had to replace 6 sticks of it, and needed to change some IC to make it working, they also replaced my camera, which i didn't know it had problems before, but nonetheless ok la not very expensive. cost me about rm400 to fix the backlight ,screen and camera, still worth it la i thought for a 6s plus kan.

But then, when i tried charging the phone after collecting the phone from the shop, the phone gets very hot and the battery gets depleted like super fast, one minute it's 100% the next minute 91%. but the battery health is showing 82% health in the battery settings.

Does yours do the same too?

Just keep as spare phone lor like me, i already changed to note 10 plus, keeping this iphone 6s plus as spare phone for surfing and stuff.. but a bit regret leh spent almost rm400 to repair but overheating and battery drain issue.. sigh..

Tried erase and restore didn't help also, most likely motherboard / cpu problem i guess?