I would suspect if you are using the phone as a screen and being outside you are going to have to whack up the brightness, especially in bright sunlight. Any phone is going to get hot in these conditions and as mentioned previously in a high temperature then depending on the phones sensitivity it’s possible to trigger an overheating warning.
Use it sensibly, try not to tax the battery, stay shaded for vision and to therefore keep the screen brightness down and you’ll avoid this. I figure the majority of users have never had and will never have this problem.