I don't know why, but I can't isolate any instances. I am reloading the model if that is what you first thought.
drawingControl3D1.IsolateInstances = selection.ToList(); // selection is an EntitySelection defined variable
drawingControl3D1.ReloadModel(DrawingControl3D.ModelRefreshOptions.ViewPreserveCameraPosition);
Sorry, my code had some strange onSelectedEntityChanged implementation which triggered a new selection every time I tried to isolate an object. It is working just fine!
https://github.com/xBimTeam/XbimWindowsUI/blob/a8de8b121dc4aa31119077fc214fd5879650035a/XbimXplorer/XplorerMainWindow.xaml.cs#L1081
I don't know why, but I can't isolate any instances. I am reloading the model if that is what you first thought.