Most of the books I read when I was younger were part of a series. Harry Potter, Redwall, The Circle of Magic, and so on were all series with which I could watch the characters develop and grow over the course of several stories and a long period of time. As I got older, there were more series to indulge in. Divergent. Hunger Games. The Xanth Chronicles.
There are relatively few things I would qualify myself as an expert on, but I think there's a relatively large portion of my generation that are very familiar with how a series works and how characters and settings can change and develop over the course of several installments.
So we also know when things go badly. We've seen it several times in the last few years. Sudden deaths of beloved characters. Finales that missed the mark. Dangling plot threads. Overuse of the deus ex machina.
I'm not going to point out any examples in particular - I'm sure you can fill in plenty of them on your own.
What I'm more interested in is WHY these endings to series that we love were so disappointing, and I think I have a relatively solid theory, based on some recent experiences and my own bias.
The first and most prevalent reason for series flops is, in my humble opinion, because the creators lost sight of what made their series popular in the first place. Sometimes it's the character dynamics, sometimes it's the worldbuilding - when the focus of the series moves away from the thing we loved most about it, then we end up in a position to be disappointed, because what we liked in the beginning is no longer present, or there only in name.
Another reason for disappointing endings to a series is that characters seem to forget what originally motivated them, without sufficient plot back up the change. It's always a bit of a shock when you get to that last big confrontation and your favorite supporting character seems to forget that the reason they were with the hero at all was something other than their romantic interest in the main character.
(I'm not bitter, I swear.)
The final reason I want to highlight here is that the creators begin to care more about surprising their audience than satisfying them. These are generally either badly-timed subversions of audience expectation or plot twists that didn't actually twist the plot. When a character dies who might have made things more interesting if they had lived, when a logical plan goes wrong for unexplainable reasons, when the solution to the Big Problem is something completely unrelated that no one has mentioned before; these are all symptoms of a creator that wants to surprise their audience, either because they're afraid of being predictable and cliche, or because they think an ending you can see coming is a bad one.
Inklings, I want to set down on the record here on the internet where everything is always true - just because you can predict the ending of a story doesn't mean the story is bad. Romeo and Juliet is lauded as a masterpiece and a classic that has survived centuries of criticism and reinvention, but it literally opens with a monologue telling you how it's going to end. Don't tell me that knowing the ending spoils the story, because that's just not true.
This isn't a comprehensive list by any means, but I think these are the three biggest reasons a series ending might disappoint longstanding fans. Do you have any to add, or examples that show the reasons above?