Weirdly enough, it does. The export plugin let’s you 1) export an existing field, choosing from a dropdown (where _numero_factura appears) or 2) fetching a custom one with a hook. I’ve tried both ways and it always returns blank, I’ve tried with other “hidden” fields and it just won’t work with _numero_factura and _id_factura, and both fields are related.
Tested this on staging and production, and nothing.