Following on from my question - is the alert status only updated when the Configuration Collection is run? This would explain the 12 hrs.
We have two types of alert in VMan based on what information we are checking. First group is evaluated against configuration data which are collected by default every 12 hours and second group is evaluated against performance data collected by default every 10 minutes. Alerts are validated once the collection jobs finish.
You can see what data is alert related by opening the alert, clicking on configuration and check the scope tab.