RadPropertyGrid's item is set as DataContext to all of its editors.. When property sets are used, RadPropertyGrid's item is of type PropertySetsViewModel. Apparently, if the property you want to add is not a product of a valid intersection between items' properties, it won't be added in the PropertySetViewModel item, so you cannot bind to it like this. You should explicitly change the Binding's source, since such property does not exist in the DataContext instance.
All the best,
the Telerik team