Inconsistent nullable feature status for WinUI3 projects

[huoyaoyuan]

Version Used:
Visual Studio 16.7 Preview 6
.NET SDK 5.0.100 Preview 7
WinUI 3 Preview 2

Steps to Reproduce:

1. Create a new WinUI(.NET 5) project from template. https://docs.microsoft.com/en-us/windows/apps/winui/winui3/get-started-winui3-for-desktop
   Building the package project requires workaround for this VS version, but building the main .NET project will success.
2. Add <Nullable>enable</Nullable> in csproj.
   The compiler will complain about that field m_Window of class App is not initialized, as expected.
3. Add nullable annotation onto the field declaration, making it become private Window? m_Window;. Then compile the project again.

Expected Behavior:
No warning about nullability present.

Actual Behavior:
The IDE IntelliSense warning goes away. But there will be warning CS8632: you can only use nullable annotations in #nullable enable context in IDE output window. The warning does not go into IDE error list window.

Other Information:
I haven't confirmed whether it's caused by WinUI or CsWinRT. But I can confirm that it doesn't apply to WPF (.NET5).

[cartermp]

In theory this should work because WinUI3 project stargeting net5 are SDK-style projects.

[huoyaoyuan]

It only repros when there's .xaml (probably requires generating .xaml.g.cs) added to the project.

[huoyaoyuan]

Opened dotnet/msbuild#6088 . This is an external issue.