What difference would flying out of another airport make? It sounds like the retailer, in this case Hermes, is handing out expired forms.
Wouldn't they manually stamp at other airports like LHR? The problem was that at CDG, if the barcode was invalid, customs officers couldn't manually stamp the forms. As far as I know in Milan and LDN, they didn't even scan the forms. They just stamped on refund forms the old fashioned way.