it seems the problem only occurs with certain document types. So far I have identified two controls that produce the problem. One of which is an older version of the WinForms SyntaxEditor with the C# Plug-in.
I thoguht the WinForms/WPF Interop scenario might be an issue, but other WinForms controls (e.g. another SyntaxEditor with custom language) don't cause any problems.
But now I am able to reproduce the issue with the sample browser. Just open the "Simple IDE" in "Docking/MDI" demos and create some C# documents until the tabs are overflowing. Now close some documents while a C# document is open. The tabs are not re-arranged. If a text file is opened, everything works fine. Likewise, switching to a C# file doesn't do anything, but switching to a text file causes the tabs to update.
I should add that we are using the following properties in our dock site: