The Septuagint Jeremiah is about an eighth shorter and differently arranged, and 4QJer-b shows that shorter Hebrew form existed. So which Jeremiah is the faithfully copied one? You can’t have two originals.

You can have two editions, which is what most scholars conclude: a shorter earlier form and a longer expanded form both circulated before the Christian era. The claim is about transmission, and both editions were transmitted faithfully in their respective lines. Jeremiah is the clearest case where the phrase the original text is more complicated than for most books, and saying so costs nothing. It also shows the tradition preserving both forms rather than quietly suppressing one.