This just happened to me for the first time (that I've been using the computer and been able to notice, at least).
I have been using my new Surface Pro (M9) regularly since it arrived on Friday.
Power was unplugged, I was using the keyboard attachment in desktop mode.
I have the event files that seem to apply to the issue (although I may not have all of them - I am unfamiliar with event viewer).
Here is the link to the event file: Let me know where to send the event file.
One thing I should add: I was using headphones and listening to music on Groove Music at the time. In the moment it turned off, the music jumped to a very loud volume, well beyond the volume at which I was listening
to music at the time. The volume returned to the original level after I powered it back on, but interestingly the volume notification showed the volume at 64 (Just to be clear - it was not playing at 64 volume now on restart, but the notification simply showed
that it was 64). Once I hit the volume down key, the display jumped back to showing the volume I actually had it set at - 14.