"Surprise! The protagonist has been working on a secret plan all along and just didn't tell you about it!"
We're inside the protagonist's head, so we know exactly what they're doing! Wrong. They've had secrets all along, and now it's time for the Big Reveal. Aren't you shocked? Isn't this a thrilling plot twist?
For me, no. Lots of people must like it, because it's been cropping up more and more in popular books lately, but it's probably one of my least favorite narrative devices. No matter how well-written the book otherwise is, if this device comes into play, I immediately mentally check out of the book, no matter how much I was enjoying it before.
"Aha! You didn't know I had a way to turn this on its head all along!" Well, I should have, protagonist, because we share thoughts.
This kind of "twist" is like a barrier slamming down between me and the protagonist. When I read, I love the sense that we're inside the characters' heads, that our perspectives are almost overlapping as the story unfolds, and these moments immediately take that away. At best, they make me go, "Dude, you didn't trust me? You kept secrets from me? WTF." But most often, I just feel cheated by the author. They were telling me the character was one way, from inside the character's own head, and actually they were another?? Even if the character is a trickster or a schemer or a liar, they shouldn't omit important details from their own thoughts.
And once that trust is gone, it's gone for good. There's no longer any point reading the book, because the protagonist could be lying to me at any time. I can't trust that the story I've been reading won't just get ret-conned for drama when convenient. No matter how much I was loving a series before, once this happens, I am done. I can try to force myself to keep reading, but I'm not going to get far. The world and the characters just don't feel real to me any more.
Of course, like all things like this, there are very, very rare occasions where it works. In Crown of Midnight, for example, Celeana is literally repressing her plot twist-y memories. She tries very, very hard not to think about any of it, so it makes sense that we only get vague hints from her before she's forced to face it. Otherwise -- well, I guess you could say the character thought about it and planned while they were "off-screen," but that feels a little cheap to me. I find it hard to believe that the perspective characters never think about these things while they're happening, especially if they're that important. If you pick one perspective to write from, in my opinion, you have to let us into it fully. And if you pick multiple perspectives, you have to let us into all of them. You can't pick and choose perspectives so you can best mislead the audience.
Basically, if the reader is going to be surprised by a plot twist, then the perspective characters have to be surprised too. Otherwise, our connection with the characters collapses, and if you're a character reader, like me, the book is never going to recover from that betrayal.