Bug: Unexpected behavior of CollectionViewSource with grouping

Description

Hello.

I'm porting a UWP application to .NET 9 and I'm running into issues with CollectionViewSource when working with grouped collections.

I use a custom grouped observable collection. It has its own Grouping<TKey, TItem> type with a Key property and a Values property (which contains the items in that group).

The porting process is stuck because of problems with my custom collection.

I created a minimal reproducible example with two variants:

1. My custom grouped observable collection (shows groups by DateTime + view model class or DateTime values)
2. Manually created grouped collection (like in issue #2001, also using DateTime + view model or DateTime value)

When the app runs with both variants present — all groups and items are displayed correctly.

But when I comment out the creation of the manual grouped collection (from issue #2001) — only the group headers are shown in my custom collection; the items inside the groups disappear.

Right column display DateTime(group key and values). Left column display DateTime(header), view model in values.

Commented:

Why my custom collection stop working?
Archive with sources: TestNet9.zip

Steps To Reproduce

1. Launch the application using the archived build. → Expected: all groups and their items are displayed correctly.
2. Close the application.
3. Open MainPage.xaml.cs and comment out lines 36 through 57 (the manual grouped collection creation code).
4. Rebuild and run again. → Actual: only group headers appear; items within groups are not rendered.

Expected Behavior

Items from my custom collection display without issues. And i don't need another two collections.

Version Info

Microsoft.Windows.CsWinRT 2.3.0-prerelease.251115.2(add as PackageReference)
CommunityToolkit.Mvvm 8.4.0

Microsoft Visual Studio Community 2022
Version 17.14.27 (February 2026)

Additional Context

No response

Start with the attached TestNet9.zip and inspect MainPage.xaml.cs, especially lines 36 through 57 that create the manual grouped collection. Reproduce the difference with that code present and commented out, then compare the custom grouped collection's DateTime keys and Values behavior in CollectionViewSource. Done means the custom collection renders both group headers and items without the second collection.