iPhone 6s keeps shutting down and restarting when phone is locked

I’ve had this iPhone since October 2018. It was purchased refurbished directly from Apple. No issues until 3 days ago.

The phone would work normally when I’m using it, but once the phone is locked and screen is off, it will restart randomly by itself. The phone would never shutdown and restart while I’m using it. It’s just whenever I set the phone down or put it in my pocket.

If left untouched, the iPhone will continuously restart until I used it again or until it runs out of battery. The problem still occurs even when the phone is plugged in.

I have attempted updating to iOS 12.1.4 (it’s currently on 12.1.3 but when the phone got to the screen with the white Apple logo and white progress losing bar, it would restart and not complete the update.

I are extremely careful with the phone. It’s always in a protective case and screen protector  and it’s never been dropped or come into contact with water.

Any help would be appreciated!