Thank you for sharing this snippet with us! Actually this behavior is the expected one. If you take a look at the default template of GridViewGroupRow you may see different elements laid out in a specific manner, following some rules. They are bound to the corresponding ViewModel and it is not straight forward to make it work using a single RadExpander. Since it is not a trivial task, I would like to suggest you to modify the default template of GridViewGroupRow and alter the ToggleButton elements to match the desired design presented in RadExpander, while preserving the UI logic of GridViewGroupRow.
the Telerik team