Jeremiah had told the residents of Judah to submit to the yoke of Babylon, to accept captivity as God’s will, to pack there bags and trust that the Lord will go with them to Babylon and bless them there. After many exiles were taken, he even told some of the survivors they could stay in the land and he would protect them. Instead they chose to flee to Egypt and forced Jeremiah and Baruch to go with them (Jeremiah 42-43). Jeremiah did maintain some contact with the Jews in Babylon (Jeremiah 51:59-64).
Evidently the Jews in Babylon took with them an edition of the writings of Jeremiah, as did the Jews who went down to Egypt taking Jeremiah and Baruch with them. Eventually in Egypt the Hebrew text of Jeremiah was translated into Greek (which had become the main language of the immigrants around Alexandria, which was where many Jews eventually settled.)
Meanwhile the scribes in Babylon continued to study and copy the writings of Jeremiah, along with the other books of the Bible.
Scholars who study the Septuagint (Greek) text of Jeremiah note that it is notably different from the Hebrew text. In particular, whole chapters (especially the “foreign oracles in chapters ) are in a different order. In addition, the Hebrew text has additional words–and the words seem to be glosses, scholarly explanations and definitions.
We can speak of a Septuagint text-type and a Babylonian text type. It gets more complicated here: some Hebrew fragments of Jeremiah found at Qumran (home of the Dead Sea Scrolls) agree with the Septuagint text-type and some agree with the Babylonian text type.
So we have to assume that there were eventually two distinct editions of Jeremiah: an Egyptian edition and a Babylonian edition.
We have already seen that there were at least three editions of Jeremiah; now we have to allow for four.
Again, these are not scholarly conjectures or hypotheses. The existence of three editions is based on the words that can be plainly found in our English versions of the Bible. It is not a dark secret, it is clearly indicated in the Bible we read. To learn about the distinct Babylonian and Egyptian editions requires some reading beyond our English Bibles–but it is based on facts on the ground–manuscripts that can be examined–not on abstract speculation.
This is the way God chose to reveal and preserve the message he gave through Jeremiah–first to the people of his lifetime, then to us also.