https://support.apple.com/en-us/HT201569
Seems it might be due to the background activity causing the phone to get warm. In my situation it was that, plus being in the car in the hot sun.
Thanks for sharing this. Mine was stuck at 81%. Did a hard reset, stuck at 83%. Did another hard reset, now stuck at 86%. My iPhone is little warm though, but not in a hot environment. I pulled it off the charger for now.