The resolution of the matter is found in the proper understanding of doctrinal development. It's not that Eucharistic doctrine was added after the fact; it's that in the time of the Church Fathers, there was no significant dispute about the Eucharist. Consequently there was no special need for St. Augustine and other fathers to invest the time in clarifying what the Church teaches on the subject as opposed to those who contradict that teaching.
[D]efenders of the doctrine of the real presence felt obliged to account for the omission of the Eucharist from the creeds, which did refer to baptism; this was, they said, due to the absence of attacks on the faith of the church in the real presence. [p. 3]Why must this be the case? Because the Church proclaims the Faith; she does not invent it.
The apostolic and catholic faith was 'one faith' because it was a faith that had been delivered once and for all and had been transmitted by apostolic tradition. Therefore it was unchanging and unchangeable, and the very suggestion that it had undergone change or development or growth seemed to strike at the foundations of apostolic continuity. Heretics could, and did, accuse the orthodox of having added such doctrines as transubstantiation to the original deposit of the faith, since it was not mentioned in any of the ancient creeds; to this the orthodox were obliged to reply that the doctrine had indeed been present from the beginning, but had not been asserted because it was accepted by everyone without question. ... Such judgments seemed to assume that there could be some sort of development or growth; on the basis of patristic suggestions about how the doctrine developed, Thomas Aquinas defended the legitimacy of the Filioque by explaining that it 'was contained implicitly in the faith that the Holy Spirit proceeds from the Father' and that it had now been made explicit. [p. 6f.]The relevant portion from the Summa Theologica:
In every council of the Church a symbol of faith has been drawn up to meet some prevalent error condemned in the council at that time. Hence subsequent councils are not to be described as making a new symbol of faith; but what was implicitly contained in the first symbol was explained by some addition directed against rising heresies. Hence in the decision of the council of Chalcedon it is declared that those who were congregated together in the council of Constantinople, handed down the doctrine about the Holy Ghost, not implying that there was anything wanting in the doctrine of their predecessors who had gathered together at Nicaea, but explaining what those fathers had understood of the matter. Therefore, because at the time of the ancient councils the error of those who said that the Holy Ghost did not proceed from the Son had not arisen, it was not necessary to make any explicit declaration on that point; whereas, later on, when certain errors rose up, another council [Council of Rome, under Pope Damasus] assembled in the west, the matter was explicitly defined by the authority of the Roman Pontiff, by whose authority also the ancient councils were summoned and confirmed. Nevertheless the truth was contained implicitly in the belief that the Holy Ghost proceeds from the Father. [ST I, Q36, A2, ad 2; emphasis added]The very history of the Councils with respect to the doctrine of the Incarnation ought to make it clear that this is exactly how such things work; hence it should not be surprising that clarification of what the Church teaches about the Eucharist was unnecessary until that teaching was challenged. The same may be said, too, concerning the doctrine of justification.