An intriguing suggestion from J.N.D. Kelly’s Early Christian Doctrines. First, concerning Ambrosiaster’s influential Commentary on Romans:
Ambrosiaster’s teaching is particularly noteworthy because it relies on an exegesis of Rom. 5, 12 which, though mistaken and based on a false reading, was to become the pivot of the doctrine of original sin. In the Greek St. Paul’s text runs, ‘…so death passed to all men inasmuch as (εφ ω) all sinned’; but the Old Latin version which Ambrosiaster used had the faulty translation ‘…in whom (in quo) all sinned’. Hence we find him commenting, ‘”In whom”, that is, in Adam, “all sinned.” He said, “In whom”, in the masculine, although speaking about the woman, because his reference was to the race, not the sex. It is therefore plain that all men sinned in Adam as in a lump (quasi in massa). For Adam himself was corrupted by sin, and all whom he begat were born under sin. Thus we are all sinners from him, since we all derive from him.’ (p. 354, quote from Ambrosiaster’s Commentary on Romans, 5,12)
So Augustine has no doubt of the reality of original sin. Genesis apart, he finds Scriptural proof of it in Ps. 51, Job and Eph. 2, 3, but above all in Rom. 5, 12 (where, like Ambrosiaster, he reads ‘in whom’) and John 3, 3-5. The Church’s tradition, too, he is satisfied, is unanimously in favour of it, and he marshals an array of patristic evidence to convince Julian of Eclanum of this. The practice of baptizing infants with exorcisms and a solemn renunciation of the Devil was in his eyes proof positive that even they were infected with sin. Finally, the general wretchedness of man’s lot and his enslavement to his desires seemed to clinch the matter. Like others before him, he believed that the taint was propagated from parent to child by the physical act of generation, or rather as the result of the carnal excitement which accompanied it and was present, he noticed, in the sexual intercourse even of baptized persons. As we have seen, Augustine was divided in mind between the traducianist and various forms of the creationist theory of the soul’s origin. If the former is right, original sin passes to us directly from our parents; if the latter, the freshly created soul becomes soiled as it enters the body. (p. 363)
There are a couple of things that need to be noted about this.
First, although Kelly places the mistranslation with the Old Latin version, the Vulgate is no different. That in part is because the Vulgate translation of the New Testament isn’t really a fresh translation (unlike the Old Testament, where Jerome did so from the original Hebrew) but a revision of the Old Latin.
Second, there’s no doubt that the Church Fathers all taught that the Fall was a disaster and left us in a sinfully impaired state. The issue here is how that disaster has been propagated. Ambrosiaster and Augustine were of the idea that Adam’s sin was directly passed from parent to child, based on the reading from the Old Latin. (Ambrosiaster’s expansion into “gender-neutral” territory is an interesting aspect of his teaching.) That has influenced many things in Christianity, from the Roman Catholic doctrine of Limbo to the insistence that infants be baptised.
As always, Bossuet has a full exposition of this idea, which you can find here.