UWP Missing back button after PushModalAsync

Description

After a modal window in UWP is closed, the back button and page title are not shown after toggling between tabs on a Tab Page. This is not a problem in iOS or Android. This is not a problem if the same page is shown using PushAsync.

Suspect line 457 of Xamarin.Forms.Platform.UWP.NavigationPageRenderer should reference Navigation.NavigationStack.Count rather than PageController.InternalChildren.Count

Steps to Reproduce

1. Run the attached program
2. Click step 1 button to add a member to the navigation stack
3. Click on Tab 2
4. Click on Tab 1
5. Click on Test 2 - Show page via PushModalAsync
6. Close by clicking on button or press the back button in the top left corner

Expected Behavior

The back button and page title should be visible

Actual Behavior

The back button is not present. The page title show be visible (there is space so the navigation bar is still present)

Basic Information

• Version with issue: 3.4.0.1008975
• Last known good version: unknown
• IDE: Visual Studio 2017 15.9.11
• Platform Target Frameworks:
  • UWP: 16299
• Nuget Packages: NETStandard.Library 2.0.3 Xamarin.Forms 3.4.0
• Affected Devices:

Screenshots

before step 4

after step 4

Reproduction Link

BackButtonMissing.UWP.zip