I'll use a different example then... The Two Towers, specifically the Battle for Helm's Deep. In the movie, the battle is about 40 minutes long, a significant chunk of the movie. In the book, I think the battle is 4 pages long?
Christopher Tolkien said the movie "Eviscerated the book by making it an action movie for young people 15 to 25." He's probably mostly right considering that the whole point of the books is describing how horrible war is, and the movies definitely glorify war.
Anyway, if the Jackson movies aren't fan-fiction, then I don't see why Rings of Power is either.