WARNING!!! iPhone XR burned me

There is no "issue". This would be not unexpected behavior if the phone where underneath you. Especially if the phone is plugged in. Make sure you put the phone on a hard surface when charging, always turn the flashlight off after use (leaving it on for long periods of time is not very good for the phone anyway) and don't leave the phone in your bed. This could happen with any brand of phone. Basic safe use ideas.

I remember, back before we took our smart phones every where, reading reports of people who fell asleep on their laptops and got burns.

Exactly! The heat needs to dissipate and when people put their devices under their pillow or fall asleep with them while in bed they can get overly hot.