Not sure if this is a bug or by design. As seen in the following images, PageTwo disappears from IntelliSense when a parameter is passed to its constructor. Of course I could just write PageTwo by hand, and it would work just fine, but this is inconvenient and might be error-prone, especially when IntelliSense is available and could accomplish the same task.
Update: This issue is not limited to Shell Page
With constructor parameter:
After removing constructor parameter:
Testing from another XAML page (PageTwo disappears):
This issue has been fixed and released.