I have tested the grouping scenarios at our WPF Demos, however, I was not able to reproduce such a behaviour. According to this forum thread you have the latest (2012.2 607) version of the RadControls. Would you please confirm that?
May I ask you when do you add the GroupDescriptor (using the code snippet provided)?
In regard to my previous reply, I would like to add that you could use ColumnGroupDescriptor instead. That way only one ColumnGroupDescriptor can be added to the GridView. In this help article you could find examples on how to use a ColumnGroupDescriptor.