I think the clue there is "even though the clock on the home screen shows 8am". Out of curiosity, what does the clock in the status bar show? If that also shows 8am then it would mean that the system clock has slipped by a few minutes. If that is correct then the clock widget showing 8am could just be that the widget hadn't been refreshed (can occasionally happen when a phone has been sleeping, I've seen that with many different Android versions), but the fact that that coincides with the alarm, which I'm guessing is provided by the same app as the widget, sounds like more than a coincidence, i.e. like the clock app itself has been sleeping.
I've not seen this myself with Oreo (different phone), but just as a wild guess what happens if you exclude the clock app from any battery optimisation?
