On my Sitecore instance I'm authenticating a user programatically like this(adapted for this post):

[HttpGet]
        public ActionResult MyAction(MyModel model)
        {
             //custom logic
             Sitecore.Security.Authentication.AuthenticationManager.Login("extranet\\"+username,password);
        }

This works well. When I call the Sitecore.Context.User.IsAuthenticated property to check whether the user was authenticated it returns true. But that lasts until the ActionResult finishes it's execution. When I make a post or redirect the user to another page the IsAuthenticated property always returns false.

For example I have a post which gets executed on the same page declared as:

        [HttpPost]
        public ActionResult MyAction(MyModel model, string anotherParameter)
        { 
               //This always returns false
               if (Sitecore.Context.User.IsAuthenticated)
               {
                    //my custom logic. 
               }
                
        }

This is what my session state looks on the web.config

<sessionState mode="InProc" cookieless="false" timeout="20" sessionIDManagerType="Sitecore.SessionManagement.ConditionalSessionIdManager">
  <providers>
    <add name="mongo" type="Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider, Sitecore.SessionProvider.MongoDB" sessionType="Standard" connectionStringName="session" pollingInterval="2" compression="true" />
    <add name="mssql" type="Sitecore.SessionProvider.Sql.SqlSessionStateProvider, Sitecore.SessionProvider.Sql" sessionType="Standard" connectionStringName="session" pollingInterval="2" compression="true" />
  </providers>
</sessionState>

As far as I can tell this is fine and should make the session last for 20 minutes.

Any thoughts on what could be wrong here?