I've read several series where it was painfully obvious that the last book was
not meant to be the last one (probably the publisher decided to discontinue it), so the author was left scrambling to try to wrap up dangling plotlines. They're always a bit painful.
Even worse, one of these series ended on book
11. At least make it 12 so it's even!!!
EDIT: Also, back in the 1980s there was a children's series that the last book left a few things dangling. I think some decade ago, I found the author's email and decided to inquire... turned out the publisher discontinued the series.
I (maybe half-jokingly) suggested that if she could get the rights back, she should rewrite and self-publish. A few years later, guess what I find available on Amazon?