I'm not a tech expert, but I do know that individual games can at most only cause game crashes. If you do have full system restarts playing on a different resolution there is something far bigger going own outside of a specific game. You already mentioned it not being GPU or CPU temperature or bad/faulty hardware, those would be my first thoughts too, but from what I know a game can never have such a big impact on your system to cause full system restarts on its own. Even when you give admin permission to a game, as far as I know it should be impossible that a game has anywhere near the access to your system that is required to reach the point where it can crash more than just itself.
Don't really have an idea what it could be beyond that. The only thing I do remember with full system restarts gaming with a high-end laptop I had was that I checked all regular stuff like drivers, CPU and GPU temperatures and all that seemed fine. Then looking through other diagnostic programs I found one that also accessed motherboard temperatures. It was the motherboard temperature that my system didn't register and once that reached ~95 Celsius, my system would crash to protect itself from getting hotter. I don't really know much of the technical side of things, but basically in my case the motherboard was placed in a spot where the ventilators didn't cool it as well as they did for the CPU and GPU, and where the fans did register CPU and GPU temperatures and reacted appropriately, they did not register motherboard temperature and let that component slowly cook itself to a full system restart.
