arhyalon has a wonderful article on her Substack about
the problem of fanfic, in which she casts light upon the problem with so many recent unsatisfying sequels, a problem that has led me to describe a number of them as "authorized fanfic." She gives us not only a great deal of food for thought on what makes even authorized sequels fail, but also some excellent vocabulary to discuss the underlying issues of being untrue to the source material -- and how they can arise even when the writer means well.
Take for instance the various Pern novels written by Anne McCaffrey's children Todd and Gigi. Although both clearly write with affection for their mother's work (unlike certain posthumous sequel authors who seem to view the 'verses in question as cash cows to milk again), but just don't seem to have the literary chops to do the 'verse justice, such that the flaws read like the sorts of flaws we often see in fanfic written by young writers with more enthusiasm than skill, who cheerfully mangle the characterization of their favorite characters to fit their daydreams -- and don't even realize that they're being untrue to the characters they love so much, because they don't know how to separate their ideas about the character from what's portrayed on the page.
And even the original authors can fall into these sorts of traps -- especially if they've come to the natural end of a 'verse, but it is so successful that their publishers are pressuring them to write more books in that 'verse. Several of Anne McCaffrey's later Pern books had a certain strained feeling about them that made me wonder whether her publisher told her that, if she wanted a contract for another book in a different 'verse she wanted to further explore, she first had to sign a contract for another Pern book. Sometimes I wonder if Frank Herbert's last two Dune books were written under those conditions, because they feel like "books written in the Dune 'verse" rather than actual Dune books.