Why DND automatically turns itself on and off in macOS High Sierra?
DND acts buggy in High Sierra when you set the time to midnight. I'm not sure if this applies to you but DND does not like 12:00 AM. Use any time except 12:00 AM such as 12:01 AM or 11:59 PM if you are setting DND time values. 

If you set DND to turn on at 12:00 AM and off at any other time, it will show as disabled the next time you look at it and the start/end times will have been changed to 10:00 PM/7:00 AM.

I spoke with Apple and they confirmed this behavior in High Sierra.