Since iOS12.2 beta 5 I have started to get the temperature warning screen pop up. Like 10 times per day. THe phone is not warm at all, some times I just pick it up after not having used it in a good while. Apple Support chat ran remote diagnistics and there is nothing apparently wrong with thehardware (my first assumption was a faulty temperature sensor).
The phone becomes very sluggish and after a few minutes the screen turns black with the temp warning message.