Hi,
when the Content of LayoutDocumentPane is empty I can dock a LayoutAnchorable with CanDockAsTabbedDocument="False" into the DocumentPane.
To repeat this issue use "TestApp" and modify the contents of MainWindow.xaml this way:

1. inside node <LayoutDocumentPane x:Name="LayoutDocumentPane"> remove all contents
2. to <LayoutAnchorable ContentId="toolWindow1"... add CanDockAsTabbedDocument="False"
3. Run.

Not expected behavior: