The PreviewTouchDown event is not raised if another window was shown before the main window

* .NET Core 3.1
* Windows version: Windows 10 (OS Build 17763.1098)
* The issue doesn't occur in .NET Framework 4.8

The PreviewTouchDown event handled at the main window level is not raised if another window was shown in another thread before initializing the main window

**Actual behavior:** Touch the "Touch me" button in the attached example and you will notice that nothing happens

**Expected behavior:** Touching the "Touch me" button should result in raising the PreviewTouchDown event and showing a message box

**Attached is a sample project demonstrating the issue.**
[sample.zip](https://github.com/dotnet/wpf/files/4650655/sample.zip)