Expander control is not narrated

* .NET Core Version: 3.0.100
* Windows version: 1903
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

The problem has already been described here: #1015. What I don't understand in the solution there is why though the expander is exposed only when not in ItemsHost Panel.

https://github.com/dotnet/wpf/blob/0166abb3a5e4ed8bbda643af98b7e1a604959c4c/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Automation/Peers/GroupItemAutomationPeer.cs#L148-L161

If I have a DataGrid with grouping, the headers of the groups are left out from the UI Automation tree and thus are practically inaccessible.