I’ve had a phone over-heat before. It was an iPhone 6. It was doing telematics (something I was working on), including constant geolocation updates. I also had it plugged in, as constant geolocation updates can drain a battery fast. But charging a phone under heavy load will make it even hotter.
I picked up the iPhone 6 to see how it was doing (mapping, acceleration, etc), and it was HOT!! A moment later a message came up warning that the phone was too hot, and was shutting down.
If that phone had been in my pocket, I’m sure I would have felt it. Not quite hot enough to burn, but it would have been uncomfortable. The iPhone case was metal, and might have been doing something similar to how the Librem 5 is planning to cool.