The SRE paper cuts list notes that Karma sessions are too short, and expired sessions can lead to dashboards silently not updating. This is a bit of a multi-lateral problem between Karma, Alertmanager itself and IDP.
Karma has (as far as I know) no way to know when a session becomes invalid, as it for the most part in our setup has no idea about the existence of sessions at all. This means that Karma can't tell when a session becomes invalid unfortunately and it's not a problem that we can easily solve without significant investment.
We are currently subject to the IDP defaults of 1 or 2 hours for session validity, but these are easily configured values.