I think it's easy to blame corporations and the need to make money from art for bad endings... but not all series with bad endings are traceable to this. Sometimes it's just... a lack of either experience or understanding on the writer's part, or lacking a good editor.
As an example, I'm going to talk about the Blackwell games. There will be spoilers, so if possible I'd recommend having played them because disappointing ending aside, they're actually really good! ...but this is a great example of an indie game with no corporate meddling coming up with an ending that... doesn't quite... work.
So the Blackwell games are about a grumpy aspiring writer called Rosa. She doesn't like people, she almost never leaves her apartment in NYC to the point that people who live there don't even recognise her. She's pessimistic, sarcastic, socially awkward and struggles to connect with people. Then she inherits the ability to talk to a dapper ghost from the early 20th century called Joey, who is tied to the women on her family line. Long story short, she's a spirit medium and if she doesn't help ghosts come to terms with their deaths and go to the afterlife with Joey's help, she will completely lose her sanity as her aunt did before her. She becomes a paranormal investigator to help this happen. ...And so a gloomy misanthrope who'd rather just be in her apartment teams up with a ghost with kind of old-fashioned ideas and must go beyond her comfort zone learning to speak to the deceased, comfort them, solve their problems and help them pass on.
Okay, so great setup, right? You'd probably expect the ending to go something like: Rosa and Joey discover that the reason Joey couldn't move on is that he did something bad. He used to be a mobster and he'd locked away those memories or had them locked away and serving as a way for ghosts to move on is his penance, but now something is happening where him leaving the world is the only way to stop some supernatural disaster. His last act of sacrifice will allow him to finally find peace. Rosa in the end must, for the sake of the world, send away the only person whose company she could rely on, the only "family" she has left and the person who taught her and motivated her to go out into the world and talk to people. She sends him on and then in the ending we see her out in the world, having finally learned to live her life, thanks to her interaction with the dead.
That's not what happens. What happens is... Rosa and Joey discover that Joey did absolutely nothing wrong in life, he was just a pure cinnamon bun tailor who got killed and ended up stuck as a spirit guide by pure misfortune. He couldn't remember this underwhelming reveal because... dying was traumatic, I guess. The baddie is going to cause a supernatural disaster and Rosa solves it by channelling a bunch of spirit energy which... kills her. She's just like "no I'm too tired of living to go on" and DIES. Oh and then Joey randomly comes back to life. In the modern day. As his reward for having done nothing wrong, I guess.
While this is definitely an unexpected ending and it's written with a lot of emotion and performed admirably by the voice talent, it's not really a thematically satisfying ending. It feels like the aim was to subvert expectations to impress people with how unexpected it was more than to deliver an ending that was a satisfying end to Rosa's storyarc.
So something for writers to remember: The best ending is unexpected, but in a way that makes you feel silly for not having guessed it based on how perfectly it makes sense in an outside the box sort of way. The ending of the novel "The Girl With All The Gifts" is absolutely amazing for this. It's one of the most perfect endings I've ever read.
One of the problems with writing serialised fiction in the internet age is that if you drop enough clues about your ending to forshadow it and make it feel satisfying, SOMEBODY will guess what it is before you get to it. You can either just be cool with that, OR you can deliberately change it to be more surprising so that people don't think you're predictable and probably end up writing a less satisfying ending. Personally I'd rather throw in a few red herrings so that there are maybe competing fan theories but be like "haha yeah, you peeps totally worked it out, well done" than be like "NUH UH! That's not what I had planned at all, it's actually not Jon Snow who fights the Night King at all it's uh- pulls out a random slip of paper from a hat ARYA! Who's predictable now, suckers!?"