So we have no known manuscripts for the first few centuries, which is indeed very puzzling.
Not really. Books were expensive - the equivalent of a year's wages in modern dollars - and Christianity was an illegal, underground, persecuted religion for three centuries. People did not simply sit around in monasteries copying Scriptures until after Christianity was legalized and made the religion of the Roman Empire, and that happened in the Fourth Century. After that, monastic brothers went about the business of copying down what they could find on proper parchment materials. (Parchment is dried, stretched animal skin: it is strong and durable.)
Before that, in the age when Christianity was hidden, there were no Bibles circulating, only letters and parts. There were no established monasteries of full-time monks copying things. If there had been, the Roman army would have showed up and fed them all to the lions. There were only fragments, and they circulated as letters. It was mostly written on papyrus, which was far cheaper than durable vellum or parchment. Papyrus falls apart and crumbles to dust pretty quickly. If it gets wet, it rots. Parchment is animal skin, so it dries off.
That is why all there is before the fourth century is fragments and pages. No organized religion stable and secure and able to write out books professionally. Christianity spread through oral teaching and some letters. It was not a Bible-based religion.
Once Christianity was legalized, THEN then the monasteries set up and copied down Scripture. And it is not surprising that the first Bibles come out of the city of Alexandria, greatest port city in the world and cite of the greatest scholarly institution in the world, the Alexandrian libraries. Those fourth century complete codicies came from there.
Truth is, every single manuscript of the Bible is different from every single other one. There are 10,000 plus manuscripts, and they are all different. Some of the differences are in spelling, or in word order, or word selection. Some contain passages that are not in others. "Scripture" means "writing", and every scrap of Scripture was handwritten for somebody's use. People copied what they had.
The differences between every ancient Scripture and every other ancient Scripture show us that God was not a Biblical literalist. The letter kills, but the spirit gives life.
Christians before the Fourth Century didn't have Bibles of their own, and only had a few letters or fragments. The religion spread just as Jesus and the Apostles had spread it: orally.