Yes. And that is the confusing part. There are many (such as papist on this forum) who decry the recognition of St Gregory Palamas by the Ruthenians. But having said that, it is particularly confusing that Rome would not recognize a pre-schism saint such as Constantine the Great--someone whom the East refers to as "Equal-to-the Apostles".
It is, indeed, confusing, but it still doesn't change the fact that, as Deacon Lance said, St. Constantine is a saint of the Catholic communion by virtue of the fact that he is a recognized, as such, by at least one of its sui juris
I don't try to understand how or why, but I accept that the Vatican does nothing to stop his veneration. Until Rome says, "No, you can't do that," (and I know it may take years for that to happen...Rome isn't as authoritarian or omnipresent as many of her detractors think), we must accept that St. Constantine is a saint of the eyes of Rome who just happens to not be, for whatever reason, on the calendar of the Roman Rite (or, presumably, the Ambrosian and Mozarabic).