There's one particular problem with a series that does not turn on its length.1 It's the problem that it's not written as a whole.
This means that in the rarest of exceptions, you have locked yourself into certain parts of the story.
This is a more frequently problem with a continuing story, not an episodic one, but any series can run into it. Namely, that the cumulative elements end up in shipwreck.
I've read three series recently where the ending flopped -- once in an episode, twice in the end of a continuing story.
Now, if the works were not series that had been published in installments, it would be simple, if not necessarily easy, to fix these, because every one of the problems has its roots long before the final installment.
One had a heroine, raised in an orphanage, hired out as a maid to a magician's house. There, her natural talent for magic was discovered. Afterwards, mysterious hints about her origin rose, and in the final book, the reveal had to be made.
Some writers can make learning that you are a child of the upper-class, that you have an inheritance, that another significant character is your aunt, a wonderful and surprising revelation. This writer didn't have that skill.
But, in particular, the revelation was not foreshadowed enough. That there was something odd about her past was made clear, but without making this revelation particularly fitting, or thematically appropriate.
Another work had two potential love interests. In the last book of the series, it was clear the the author had spent so much time on the main plot, and so little on the love subplot, that the heroine really had no reason to choose one over the other.
So she didn't. The problem with that was that her solution of staying with her beloved father founders on two problems, because it was presented as a permanent solution.
The first is that she was the only heir to the throne. The next closest people went into exile for their offenses against the throne. No kingdom would countenance her remaining unmarried; she needed an heir. (And we are assured that she lived on to be regarded as an excellent queen.)
The other was, of course, that her father was mortal. She would be left an orphan in due course. She, actually, owing to occurrences in the story, would be left an orphan for a long time. She was magically granted longevity. (True, that complicates the heir question. If she abdicated, perhaps?)
The only way to fix this one would be to dig deep. Excise the love interests entirely, or perhaps just excise the hints that they were love interests, so that the choice is unnecessary.
Conversely, expand the subplot so that her need to choose is dramatized and given force. If necessary alter one or both love interests to make the choice more suitable.
A third work had a mysterious conspiracy, looking for women with a certain magical talent. Desperate efforts were put into trapping them with a woman of that talent, with questions of about the justice of using her as bait and a counter-observation that they were looking for her, whether she was used as bait or not, and as bait, she had protections that she did not have without it.
The actual conspiracy -- well, while it did show the most junior members because they were put as the guards after her capture, while their superiors did things, but they were young to the point of childish.
At that, her capture allowed her protectors to track her down to their lair. They were easily overcome far too easily. If their only defense was secrecy, they would not have been that sloppy, and in their own stronghold, they would have ample opportunity to strengthen their defenses.
Furthermore, the only problem with regarding their grievance as legitimate was that the set-up they complained about did not hold together. It would produce problems that she would already have seen at least some effects of. At the very least, other people suffering from the effect of the edict they complained of would have tried more reasonable means.
An appeal to the monarch would have likely resolved it.
Their entire plan to fix it would not have worked, in that it both would not proceed as they intended, and would not have solved their problem if it had proceeded. The story did not suggest, however gently, that they had not thought it through.
Nor was there any suggestion that the actual plotters had more nefarious and well-thought out plans, with the guards having been drawn in by their grievances and lack of thought.
The conspiracy was, in fact, a thundering anticlimax.
Thundering anticlimax can work. It is very hard to pull off, but I have seen it done. Where mysterious energy beings, feeling safe, revealed themselves as ordinary humans with clever disguises, ordinary would-be conquerors -- but still powerful, which is important. Villains must still manage to be villains and perilous enough to be the climax to the story, or episode. A foolish plan that is a grave danger to the main characters will work.
It is conceivable that the conspiracy could have fit the mysterious manner in which they were introduced, but that would have required more careful construction. Using their appearances and disappearances as a fighting power, perhaps.
If you want the childish conspirators, they are the gullible henchmen that the true villains are using as pawns. It is possible they could be of some assistance once the plot is in full flower, but more a problem in that they had to be protected from those who regarded them as pawns.
Even if the higher-ups were not so powerful as the forces brought against them, they needed to have something to deploy to make the climax the biggest issue. And if this required they use something besides their ability to move about so rapidly it was hard to catch them, that had to be foreshadowed.
Toning down the conspiracy would have derailed the story, but if the ending didn't work, there was also the possibility of excision, of a different episode occurring in its place, except that since the work was serially published, it was locked in.
Such are the aesthetic perils of serial publication.
The Series Forever And Ever And Ever
Some series are long stories, which, like all stories, come to an end.
Haha, yikes! I won't ask what stories these were. From my fanfiction years, I learned to never, ever publish an incomplete story. I wrote the whole thing, edited it, THEN serialized it. It saved me from a lot of the mistakes you mention here, like having to go back and foreshadow things you didn't figure out until the end. It also saved stories from dying in the Great Swampy Middle. Highly recommend.
Reminds me of the Folding Ideas discussion of Fifty Shades of Gray and how its serialism pokes through in the plot of the book and movie. Before that, I'd never thought about how the form of publication could impact the structure! Writing serially sounds terrifying to me tbh