Hard to say without having the device in hand and you willing to allow some probative research (and from my screen replacement days, people tend to get twitchy

Hard to say without having the device in hand and you willing to allow some probative research (and from my screen replacement days, people tend to get twitchy off you have to remove the person from the phone for longer than an hour, heck I was the same when I last had apple replace my battery). It could be a number of things, a bad battery not retaining charge as well as it should. It could be that one of the other processors on the device didn’t get the message to power off (which is my guess). If it’s a continuing issues I suggest backing up the device, factory resetting it and then reflashing iOS (you can use ipsw.me or 3utools to download the ISPs file needed, if you used 3utools, you can then use that to flash it or (iirc) hold down shift/option as you click the restore button in iTunes (with the phone connected) and then select the ipsw file downloaded. Restore the phone with your fresh backup and try again. Hopefully that will kick anything that was hanging around after power down to get the message.