Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 22/35. I'm trying to provide the answers in a common format, for easy reference.
22) Why do Protestant scholars recognize the early Church councils at Hippo and Carthage as the first instances in which the New Testament canon was officially ratified, but ignore the fact that those same councils ratified the Old Testament canon used by the Catholic Church today but abandoned by Protestants at the Reformation?
They don't ignore the treatment of the Old Testament canon by those councils.
1) Hippo and Carthage actually had two different Old Testament canons.
2) Until the time of Trent there was not a single view about the canon of the Old Testament. During Luther's lifetime, Cardinals of the Roman Catholic Church affirmed the shorter canon that was promoted by Jerome and has been maintained by the "Protestant" churches.
3) Athanasius' 39th festal letter, which came before the councils of Hippo and Carthage is generally viewed as the first official complete list of the New Testament canon (it comes decades before the councils of Hippo and Carthage). Steve Ray conveniently ignores this letter, as well as the fact that it affirms that the number of Old Testament books is 22 (as per the Hebrew grouping) and identifies books that more closely correspond to the Hebrew/"Protestant" canon than to the canon adopted by Trent. In fact, Athanasius explicitly rejected as "Scripture" some books that Trent included.