Were Malachi and Isaiah normally bound or grouped together? This is probably not a surprised to scripture veterans, but noticed that Mark 1:2-3 is a quotation from two separate sources, Isaiah and Malachi, but it is ascribed only to Isaiah by the author of Mark. That leads the question above.
Rick Watts, a New Testament Professor at Regent College, wrote this article in response to a reader's question raising the same issue.  It turns out the discrepancy was already noted as far back as in the 4th century A.D. by St. Jerome, the famous Latin Vulgate translator!

It looks like the earliest manuscripts left "in Isaiah" intact while later manuscripts made the adjustment, thinking that Mark made a mistake, as St. Jerome observed "O Apostle Peter, your son Mark, not in the flesh but in the Spirit, has made a mistake."  So those later manuscripts inappropriately tampered with the text and changed it to "in the prophets".  It looks like KJV used a later manuscript while modern translations like NASB and ESV used an earlier manuscript.

Dr. Watts wrote that there was a pattern in the Gospel of Mark, when bringing together separate texts to form a composite citation that "mutually inform and interpret one another" (in this case Isaiah 40:3 and Malachi 3:1).  Mark, following common 1st century practice, would cite only the most important author that provides the "primary interpretative horizon", which in this case is Isaiah.