the leader tracks session expirations by calculating when a session will timeout and then periodically checking to see what needs to be timed out based on the current time. this works great as long as the leaders clock progresses at a steady pace. the problem comes when there are big (session size) changes in clock, by ntp for example. if time gets adjusted forward, all the sessions could timeout immediately. if time goes backward sessions that should timeout may take a lot longer to actually expire.
this is really just a leader issue. the easiest way to deal with this is to have the leader relinquish leadership if it detects a big jump forward in time. when a new leader gets elected, it will recalculate timeouts of active sessions.