I came across a jarring statement in Unfinished Tales, in the chapter "The Palantíri":
It must however be noted with regard to the narrative of The Lord of the Rings that over and above such deputed authority, even hereditary, any "Heir of Elendil" (that is, a recognized descendant occupying a throne or lordship in the Númenórean realms by virtue of this descent) had the right to use any of the palantíri.
This passage seems to occur "in-universe", which is why it surprised me. I don't recall coming across a reference to The Lord of the Rings as such in Tolkien's other works, in an in-universe context. I was also under the impression that Frodo's title for the story we know as The Lord of the Rings in The Red Book of Westmarch was considerably more wordy, something like The Downfall of the Lord of the Rings and the Return of the King.
Would Frodo, Bilbo, or Sam have called the story The Lord of the Rings? If not, did Tolkien simply make a mistake here?