PushAsync stops working in Xamarin Forms when App.MainPage is re-assigned

Using Xamarin Forms I find when following certain instructions I find PushAsync stops working. It appears to be linked to re-assigning the App.MainPage object. I need to understand what I am doing wrong. Any help is much appreciated.

I've attached a zip of a simple project which demonstrates the issue I have.

Application overview

Separated into two areas, logon and main application. On startup the App.MainPage is set to a Navigation page containing ContentPage to capture username and password. When the user clicks Next another logon page is displayed which could be used to capture three random letters from a memorable word orr phrase. Once the user completes this page and clicks Logon the App.MainPage is updated to a new MasterDetailPage containing the Menu and the welcome page.

Issue

What I'm finding is after I've logged on and see the welcome page, if I click the Logout button on the menu and am brought back to the initial Logon screen when I click Next nothing seems to happen.

Steps to reproduce

1. Start application (assign App.MainPage to Logon NavigationPage)
2. On Logon (1 of 2) page click "Next Page"
3. On Logon (2 of 2) page click "Logon" (assign App,MainPage to Main MasterDetailPage)
4. On Welcome page click "Menu"
5. Click "Logout" (assign App.MainPage to Logon NavigationPage)
6. Back on the Logon (1 of 2) page click "Next Page" again and even though you can debug through, the page isn't displayed.

Any guidance would be much appreciated.

Thanks in advance