This started happening to me also. The last two days my morning alarm hasn't gone off. It seems that a recent update has really screwed up my phone. If I don't have an app running in the background I almost never get notifications.
same problem with me. Alarm does not go off.
I am waiting for the November update. May be it fixes things. Usually my Nokia 6 gets update on 29th of every month. Although after not working for almost weeks, my alarm was working this morning again, but there is definitely some issue with the background app management with 7.1.2 update. Many times my mails aren't syncing on Outlook. So I've to open the app to let them sync. HMD Global acknowledged a problem and promised a fix for background app management for Nokia 5. I hope an update fixes it on Nokia 6 too.
First make sure the Clock app is updated to its latest version from the Play store?
Next I suggest, without any guarantee of success:
Settings > Notifications > Menu (the ellipsis top/right) - has an option to 'Reset app preferences'. - Please note this also enables previously disabled apps, and you may have some work to do in other apps afterwards!
Settings > Apps > Clock > Storage - has options to clear data and clear cache.
(This is an option for any misbehaving app).
I can't provoke any 'Missed alarm' notification on my TA-1021 Android 7.1.2 - it has worked reliably during both the October and November updates.
The defaults In Settings > Sound and the sound alerts in some apps changed to a numerical value earlier, causing it to become silent, but has worked for me since setting it to proper sounds.
I hope you get it sorted out.