"If fans guess your plot twist, it means you've laid the groundwork for it to be believable. The point of a twist is to enrich a story, not feel superior for outsmarting your audience."
I can see both sides of this, but honestly I'm more inclined to agree with the above quote than not.
Firstly, because everyone's mind works differently: if you have a large enough audience, the odds that not even ONE of them will have any idea what you're gonna do next are ridiculously small. Some may just be clever enough to read the subtle clues you leave along the way; some may just go "hey, you know what'd be cool" and guess it by chance. Stories are based on logic, not randomness; there are only so many directions they can take and still make sense...
All that is to say, there's no such thing as a foolproof plot twist, so there's nothing wrong with someone being able to guess it. As long as they're in the minority, not the majority...y'know, so it can actually qualify as a plot twist.
Secondly..."un-guessable" plot twists aren't always all they're cracked up to be. I recently re-watched a series that ended with a plot twist so insane it fooled me the second time, too. The events of the entire episode ended up being just a ruse to trap the main antagonist...and at the end they actually went back through the story to show the viewers how it really went down.
That's supposed to be the fun part, right? Looking back at the story and seeing the clues you didn't see the first time...but, at least in this instance, I didn't like what I saw.
The 'tricks' just felt too unfair...sure, the MC was acting out of character, but not totally out of character...there was at least one episode a season where she got vindictive and the other characters had to talk her down; why should we be suspicious of this behavior now??
Sure, we never actually saw the deaths of the characters that were "assassinated", but what we saw looked enough like death to fool anyone (that was the whole point). Guy gets strangled and passes out, guy gets his throat slit and collapses, guy is forced to drink poison, convulses, coughs up blood and goes limp...how are we supposed to think "oh, they're not actually dead?" Like, how??
It felt like a series of instances in which the camera just conveniently switched scenes to keep us in the dark (aided by the introduction of a couple new death-faking gadgets apparently created specifically for the episode...god knows where the characters even got them from), not a trail of clues that anyone could reasonably be expected to follow.
Like any good story element, I really think a plot twist needs buildup: you gotta have at least one moment where the viewers go "wait a minute...that's kind of weird" that they can use to seed a reasonable doubt.