It's not written in Malachai either. It's a compound or conflate reading. That's why the KJV uses "prophets" and not "Malachi".
Check it out:
Mark 1
(2) As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee. (3) The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight.
Isaiah 40:3
The voice of him that crieth in the wilderness, Prepare ye the way of the LORD, make straight in the desert a highway for our God.
Malachai 3:1
Behold, I will send my messenger, and he shall prepare the way before me: and the Lord, whom ye seek, shall suddenly come to his temple, even the messenger of the covenant, whom ye delight in: behold, he shall come, saith the LORD of hosts.
See? It's a conflate reading. Now that we've established that, we have to ask, how would a Jew writing in the first century find the source of a compound quote? Where would you look first? Remember, there is no internet or search tool. It was simply the custom at the time to cite the major prophet. It's clear from the evidence, that is, the earliest manuscripts reading "Isaiah" and the later manuscripts reading "prophets", plus surviving extrabibical Jewish writings, that a later scribe who didn't understand how Jews would cite scripture changed it to correct an error he saw. Jews reading Mark at the time it was written wouldn't have seen this as an error. There is no other reason for the earliest manuscripts to read "Isaiah" in Mark 1:2.
Is the old testament reliable?
The Talmud was written after the New Testament was closed. So, I'm not seeing your point here. Mark was a Jew living in the first century and became a Christian. He learned to write like a Jew would in his time. The fact is, if you tried to call Mark wrong and point this out to him the day after he wrote it, he'd be very confused and say something like: "What are you talking about? That's how we cite it." That the Talmud, written after Mark's death, says terrible things about Our Lord does not have any relevance as to how Jews would cite scripture at that time.
Well, if you want to know how or why scripture citation was done differently in the first century than we would do it in the twenty first, you kind of half to refer to Jews from the first century, silly.