Interestingly, even in the ancient Church there was some disagreement about the authorship of the works attributed to St. Dionysius. For example, St. Photius (died c. 893 CE), in his Bibliotheca, mentions a book that he had read on the matter:
Read the treatise of Theodore the Presbyter, in which he undertakes to prove the genuineness of the works of St. Dionysius. The following arguments against it are refuted: (1) If they are genuine, how is it that none of the later Fathers cites them or quotes any passages from them? (2) How is it that Eusebius Pamphili, in his list of the writings of the Holy Fathers, does not mention them? (3) How is it that these treatises describe in detail rites and customs which only became established in the Church gradually and after a long time? The great Dionysius, as is clear from the Acts, was contemporary with the Apostles [whereas most of the institutions described only became established gradually and in later times]; it is therefore improbable (says the objector), or rather a clumsy fiction, to assert that Dionysius could have undertaken to describe institutions which were not fully developed till long after his death. (4) How is it that a letter of the divinely-inspired Ignatius is referred to? for Dionysius flourished in the time of the Apostles, whereas Ignatius suffered martyrdom during the reign of Trajan, and wrote the letter referred to shortly before his death. Theodore endeavours to solve these difficulties and does his best to prove the genuineness of the treatises.
But I would agree that the Orthodox Church didn't doubt the sainthood of St. Dionysius.