Meteor Session and this.userId return NULL


I’m new to Meteor and working on react native app. We have some subscriptions in app. A rare problem we encounter is sometime server sends empty data to subscriptions. And when this happens the this.userId is null.
At this stage if we kill the android app and restart it again, it auto logs in and get data from server correctly.

My understanding after reading docs and forums is , this.userId is like a session variable and it will have value when there is a userid attached with the current session. So when exactly a session ends? and how it reconnect session if we kill and restart android app?