I'm too facing this problem of auto restart in my Samsung a51. Also the phone hangs Sometimes when I'm using both WhatsApp at the same time. When I try to close the WhatsApp, the keyboard remains on the background without closing and after that the phone restarts. This has happened atleast 3-4 times with me.
I don't know why you are facing this problem !!! I also have a51 but I don't face any of these problems !!! You should check , if you had installed any harmful apps from google !!! or check there's an option in setting of auto restart !!! may be that is on that's why you have this problem !!!
