Subclassing ScrollView won't allow custom templates

[dotMorten]

Describe the bug

I was trying to subclass ScrollView, and while that works fine, I'm not able to change DefaultStyleKey to my own style from Generic.xaml. When I set it to my own type, OnApplyTemplate is never called and the control fails to render.

Steps to reproduce the bug

1. Either download and run sample app and skip to step 5: ScrollViewTemplateBug.zip
   or:
2. Create a new templated control and inherit from ScrollView.
3. Set the DefaultStyleKey to the type of the control
4. Define a matching style in Generic.xaml (it can just be the default scrollview template with the type parameters changed).
5. Run sample and observe OnApplyTemplate is not hit in the custom control (and attached sample renders the red background)
6. Remove the "DefaultStyleKey" setter in the constructor and notice that OnApplyTemplate is not hit and the template from the ScrollView baseclass is applied instead (attached sample now renders green).

Expected behavior

The custom template is applied so that we can easily attach additional behavior to scrollview

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.6.3: 1.6.241114003

Windows version

Windows 11 (24H2): Build 26100

Additional context

No response