When building a Net5 control library, the default template is not being applied

Describe the bug I have a class library with a custom control that has a generic.xaml template. The template defined in the class library doesn’t get applied, but you have to re-apply it in the application as well.

Steps to reproduce the bug

1. Unzip the repro: MyControls.zip
2. Set the package project as startup.
3. Run the application.
4. Observe that OnApplyTemplate in \MyControls\MyTemplatedControl.cs isn’t hit.
5. Manually define a control template in app.xaml and observe that it is now hit, demonstrating that the template defined in Generic.xaml isn’t working.

Note: This might be an artifact or related to #4453

Expected behavior Control template defined in the class library works (this works if the target is UWP).

Version Info NuGet package version: [Microsoft.WinUI 3.0.0-preview4.210210.4] Visual Studio 16.9.0 (I’m fairly certain it worked in the last preview of Update 9)

Windows app type:

UWP	Win32
No	Yes

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop
Xbox
Surface Hub
IoT

Additional context

[billhenn]

Wow this is a frustrating one. I just ran into it myself. Sure hope this is fixed ASAP.

[azchohfi]

I believe this might be a regression on 16.9 stable? I started seeing the same issue on my machine today. I will investigate further tomorrow.