Has to run App.OnStart method directly. Why?

Good day. I've got unexpected behaviour in my app. In solution use two projects - Android and UWP. So I try to get authorisation from server, so use async methods. Auth starts before main page loads, and I start it in App.xaml.cs. So here is the code:

public App()
        {
            InitializeComponent();
        }

protected async override void OnStart()
        {
            bool isUser = await accountManager.InitAsync();
            if (isUser) MainPage = new NavigationPage(new MainPage());
            else MainPage = new NavigationPage(new RegisterPage());
        }

In accountManager.InitAsync(); I restore user from App.Dictionary and make POST request to a server, to check user in DB.
When I start UWP project it works as I thought - I get answer from server and get needed page. But when I start in in Android device no page is loaded - though accountManager.InitAsync(); sends post request and I get answer. And when I call OnStart from App(), it works as planned:

public App()
        {
            InitializeComponent();
            OnStart();
        }

protected async override void OnStart()
        {
            bool isUser = await accountManager.InitAsync();
            if (isUser) MainPage = new NavigationPage(new MainPage());
            else MainPage = new NavigationPage(new RegisterPage());
        }

But in this case I got 2 calls to a server, not critical, but wrong, as I suppose.
So why is it so? Should I change the logic to use async methods at app start? And what I do not understand in OnStart functionality?