Which novel in a series feels like a fuck you letter to the fans?

Last novel of the Witcher Saga really dropped in quality.

That story peaked in the penultimate book The Tower of the Swallow when Geralt was searching for missing Ciri, and was companions with Dandelion, Cahir, Emiel Regis and Milva. And also the set up of Emhyr var Emreis as the main villain before we know who he is. Bounty hunter Leo Bonhart and his search for Ciri was just epic as well. He was so badass and so evil. It was just so much fun. I couldn't wait to see how the story continued.

Then I read the final novel, The Lady of The Lake. It read like author Andre Sapkowski did everything he could to avoid a more traditional ending to a fantasy series. To the detriment of the book. There were just long scenes of Geralt fucking, the character given up on his quest throughout most of the book. Then it just ends in the least climactic way possible with Geralt seemingly intentionally getting himself killed.

What other sagas ended so badly it seemed like a fuck you letter from the author? Or it could even be sequels.