Here's an excerpt on it from a book on celibacy

The Germal scholar Friedhelm Winkelmann has studdied this question recently and has definitively concluded that the episode is an invention. He offers the following reasons. The person of Paphnutius is found only at a much larger stage, his name appearing only in later manuscripts of the Acts of the Council, and in those from the fourth century he is known only as a confessor of the faith; it is only later hagiographic legends that begin to consider him a thamaturge and Father of the Council of Nicaea.

But the most telling argument against the authenticity of this epiosode seems to be the fact that the Eastern Chucrh itself, which should have had the greatest interest in it, either did not know of it or, because Eastern church leaders were convinced that it was false, did not have a record of it in any official documents they used. Neither the polemical writers of clerical celibacy nor the great commentators of the twelth century on the Syntagma canonum adauctum - the great code of law of the Eastern Church stemming from the Council of Trullo - (Aristenus, Zonarus, Balsamon) made mention of the story of Paphnutius, although it would have been much simpler than resorting to the manipulation of history texts which were in fact quite well known.

It was only in the fourteenth centry that this account appeared in the Syntagma Alphabeticum of Matthaeus Blastares, who, however, seemed to have considered it of interest for the East only after its appearance in the Decretum of Gratian. In the West, this falsification was accepted uncritically, at least among canonists, who in part based the Eastern Church's particular discipline of celibacy on it. Rather interestingly, the Secoond Council of Trullo did not refer to Paphnutius in officially establishing that the rule of celibacy would continue to remain in the future.