My new Galaxy Book Flex (15in.), have only had it for a couple of weeks, has been turning off when left idle. Once it goes off, pressing the power button or any other key will not make it turn back on. It will only turn back on with a hard reboot.
Not sure if it's a software issue, hardware issue, but this happens even with a full battery so don't think it's a battery saving feature, especially since it needs to be rebooted.
I'm sorry to hear that, do you by chance have the laptop set up to sleep after such amount of time? If you do try turning it off to see if this still occurs?