I had about 2 or 3 random reboots on a warm day when I carried the FP3 in a pouch, and otherwise used Maps and left it on or something like that. The device was very hot! It was a few weeks ago, still running Android 9. I don’t use microSD as of now in FP3 ("64 GB should be enough for me 
").
I really don’t want to sound like an apologist, but the situation wasn’t by any means normal (it was warm, I had the device in a pouch, and I left Maps on).
Given I recently read this trick I wonder if it actually might be EM triggered from hot weather.