The property changed of the property bound to IsExpandableBinding (property of RadGridView) is not reflected in the row. To reproduce this the IsPropertyChangedAggregationEnabled property of RadGridView should be set to True (which is the default value) and the view model property (bound to IsExpandableBinding) should be updated consequently with other properties of the view model. For example:

myRowModel.Name = "New name";
myRowModel.IsExpandable = false;

To work this around, use the IsExpandable property of GridViewRow. You can data bind it using the RowStyle property of RadGridView. Or alternatively, set the IsPropertyChangedAggregationEnabled property of RadGridView to False.