Periodic Event Frame(s) Not Closing

We have a periodic event frame scheduled for every hour; the starting trigger is the hour is either 8:00 AM or 8:00 PM, closing trigger is the house is not 8:00 AM or 8:00 PM.

The element is NOT part of a template.

The purpose of the event frame is to send a notification 8:00 AM and 8:00 PM daily to confirm the system is working.

Last night, the event frame didn't close, and, as of this morning, it is still open; the analysis should be firing every hour, but seems to have been lost.

Has anyone else experienced this, and, if so, outside of a manual backfill/recalculate or restarting the system, how did you resolve it?

What might be the underlying cause?

 

We have had other templatized event frames show similar behavior; they were resolved by backfilling/recalculating, but we'd like to discover the WHY.