Since the time of purchase and configuration I have found myself scratching my head over and over at what is going wrong in my system. It has been crashing and freezing almost since day 1. There is no consistency to how long or what I am doing before the system crashes or freezes. I could be playing AAA titles to just plain sitting at windows login screen and the system will crash. Upon the crash or freeze when it comes to rebooting a system the soft switch on my case does not reboot nor power down at all. Even long pressing the switch for about 45 seconds does nothing. it makes me cringe every time but I have to physically flip the switch on the PSU to get it to turn off. Also, before this happens the monitor typically goes black.
I had an almost identical issue when I put together my Ryzen build. I too have an Asus board, though not an identical one.
My issue ended up being that I had Asus AI Suite III installed, which was causing these freezes to happen. If you have that installed, uninstall it and see if the issue persists.
I would also update your BIOS and you could also try reinstalling Windows.