Same issues here with 8.1 on my L920. (If you want the short version - I think it's the quite hours integration with Cortana. Turn her off and all is good)
I had about 7 different alarms set prior to the upgrade. The two main recurring alarms: Weekly for work set at 4:01am, weekends set for 6:30am. All alarms were programmed prior to the upgrade and remained in the phone memory when I completed the Upgrade to 8.1.
After the 8.1 update I set up Cortana with and quite hours. Checked my Alarms (all looked good) and went to bed. Here's the history (and where the fun began). Throughout this process I always made sure all of my separate volume levels were set to 5 since it's not very clear which volume controls alarms.
(suggestion: Alarms should have an "In-App" volume level that is not linked to any other volume control)
1. I set up Quite hours from 10pm to 5:30am.
results: No alarms, no notifications, etc. during the set quite hours. No Alarms during and after quite hours turned off.
2. I then set up Quite hours from 10pm to 4am (thinking there was a minute between quite hours turning off and my alarm coming on)
results: No Alarms, But Notifications work as intended. No Alarms during and after quite hours turned off.
3. I then deleted all Alarms and turned off quite hours, (standard power on/off of phone). Then set up quite hours for 10am - 3:45am. then set up only my two reoccurring alarms for 4:01am weekly and weekend 6:30am
Results: alarm work first night of two (did not turn off phone between 1&2 night). but notifications worked as intended after quite hours turned off, etc.
4. I then deleted all Alarms and turned off quite hours, (and completed a Hard reset (Vol- / power). Reset my two reoccurring alarms for 4:01am weekly and weekend 6:30am and did not turn on quite hours.
Results: alarm work first night of two (did not turn off phone between 1&2 night). Received Notifications all night as anticipated.
5. I then deleted all Alarms and turned Cortana completely, (and completed a Hard reset (Vol- / power). Reset my two reoccurring alarms for 4:01am weekly and weekend 6:30am and did not turn on quite hours (Not that you can. Cortana must be turned ON to use quite hours, which is the dumbest line of code/programming ever written).
Results: Alarms work over multiple nights. Including powering the phone off/on and not touching alarm settings at all.
Sorry Cortana I guess you really are Beta and Waking up for work is more important – Bye bye,off you go.