Hi Kulbhushan,
At the moment when the portal session expires and browser window is closed - Is the data manager refreshed? PLease check the below excerpt from Data Manager reference guide - "Retrieve notifications delay in seconds -The length of time to wait after a record change notification is generated by MDS before Data Manager retrieves it. This delay can improve Data Manager performance as it allows Data Manager to retrieve notification in batches rather than one-byone"
Thanks, Ravi