Put bluntly: The Septuagint, as a whole, is widely regarded as a mess in preserving the overall biblical account—hence the widespread use of the mt as the basis for translations of the Hebrew Old Testament.
Generally speaking, textual scholars regard the more difficult text as the more original. What then happens, is that later scribes come along and alter the text for the sake of harmonization. There are plenty of manuscripts amongst the Dead Sea Scrolls, besides the proto-Masoretic manuscripts, that show clear attempts at harmonization. If the p-MT has tidied up the mess, it renders itself a candidate for the label “a harmonization”. This makes it a later manuscript because it modifies an earlier text.
The Septuagint has different content for some biblical books, and different combinations of books compared to the p-MT. Judging this as an error by comparison to the p-MT is to presume that the p-MT is the criterion against which it should be judged. But this is to assume what needs to be proven.
As an exercise in discernment compare the book of Esther in both the Septuagint and the p-MT. Esther saves the exiled people of Judah under Persian rule from annihilation. Is this act of salvation entirely accomplished by the womanly wiles of a Judahite woman, or did God have a hand in it? You will look in vain for mention of God in the p-MT tradition of Esther.
While we speak in general terms of translations of Hebrew into other languages, we have to consider what changes took place in the Hebrew language itself over centuries. For an example in English, we can read earlier forms of English in “Canterbury Tales”. While Canterbury Tales and modern English are all “English”, modern readers may well need a translation of Canterbury Tales to understand it. Modern English has been subject to many influences since the time of Canterbury Tales.
Hebrew is no different. It is a Canaanite language. But when the Judahite exiles in Mesopotamia returned to Judea, they had had years of thinking and speaking in another language - Aramaic. An indication of how pervasive this cultural influence was is found in the fact that they returned writing Hebrew in Aramaic script - a practice that continues to this day. An open question is, "to what extent was their understanding of Hebrew influenced by their own Aramaic language, and thus, to what extent did it involve a translation from an earlier form of Hebrew to their contemporary understanding of Hebrew.
I may be wrong, but to the best of my knowledge, the only manuscripts found amongst the Dead Sea Scrolls written in paleo-Hebrew script - the more original Hebrew script - are the pre-Samaritan scripts. The point I am making is that the p-MT manuscripts found amongst the Dead Sea Scrolls and written in Aramaic script may have involved as much of a translation - from an older form of Hebrew to an Aramaic influenced Hebrew - as was involved in the Septuagint.