MS Teams Application HTTP Context Session Changed when Postback

 We have a Provider-hosted SharePoint Add-in with several pages.

We put them on a MS Teams App as a tab by using App Studio.
The pages works well at first on MS Teams Desktop but recently users report that the issue persist while using Teams Desktop. 
After adding some debugging logs to the add-in, we found that the page session is null after postback and result in the application failed to get the SharePoint Context only when using MS Teams web and Desktop.

To replicate,

create a provider-hosted add-in and install to a SharePoint site

create a blank page with a button that will trigger postback in the add-in.

create the Teams app using App Studio and add the page as Tab.

Test and install the app

Open the tab and click the button using MS Teams web

Meghana-MSFT