Very very strange, I have the identical AMD equivalent to your board (mine is X370-F Gaming) and mine did the exact same thing as yours only a few weeks ago.
I put mine down to a hardware change as I had installed a new GPU when it started and after speaking with Asus support twice about the issue we concluded it was more than likely a UEFI bug causing it.
What's even stranger is it stopped doing it after around a week without me doing anything. Obviously I tried moving and reseating RAM and moving and reseating the GPU but neither did anything then one day I woke up, switched on my PC and it just worked and it's continued to work normally ever since.
