Mi8 with 9.9.6 here. Yesterday I put my phone into my backpack for about an hour. When pulling it out again it was crazy hot and in the process of booting up (Mi logo and the three dots, seemingly for while already). Long pressing the power button lead to a reboot with the same result. I repeated the procedure like two or three times, then finally it did boot successfully (maybe due to cooling off). Battery had dropped by around 30% (from 5x to 2x), but otherwise I could not figure out what had caused the enormous heat-up. Since the incident it is running just fine.
Should I be worried?
edit: 9.9.3 of course