Yes same problem occur in my mobile. i am set the alarm at night but in the morning it will disabled and not ringing, and when i am open the clock application in the morning it will again enabled.
Other issues i am face that, i am not getting any notification of application(hangouts, ymail, amazon, etc.) until my application is not running in background.
Have you tried removing all alarms and re-adding them?
I've updated to Android Oreo 8.0, removed all previous alarms, added them back and they work absolutely fine.