@bk69Some people spotted it coming (which is fine - there is foreshadowing of various sorts), many didn't.
I disagree that it should be in the description. Nothing in the description is incorrect, nor does the twist change the nature of the story, affect the codes, etc. If the twist 'broke' a character or the plot, that'd be different.
I received no complaints from anyone who was upset because it wasn't in the description. Perhaps someone was. In general, in any long, complicated story, something is likely to happen that surprises the reader, and that's OK, as long as it doesn't violate the codes or change the nature of the story in a fundamental way.
I believe that the story reads differently if one knows what's coming than if one doesn't, and that it's better for things to be confirmed at the point where the first-person MC becomes aware of them, rather than the reader spending months of time in the story knowing something fairly important that the narrator is not aware of.
I'm not basing the story on the 'twist' - it's not 'The Sixth Sense' - but, to me, it works better as a surprise.
I'm not a big fan of authors trying to surprise readers.
I can think of dozens of counter-examples, really. I don't disagree at all that 'if the story is good and the characters compelling, the reader will be interested without being shocked'. I can reread many stories knowing the (former) surprises and have a great time. That said, if the story is good enough without the surprise, it might be even better with it.
'The Sixth Sense' is a very good movie whether or not you've seen it before. However, it's a better movie if you go into it knowing as little as possible. To avoid spoilers, that's not just 'the twist' that I'm mentioning. An awful lot of movie happens before you even know what's going on with Cole; he doesn't say his famous "I see ..." line until, if I recall correctly, nearly an hour in. Until that point, the viewer has no idea what's going on with him, just that something very weird is going on.
If you know that line, if you know the twist, it's still good, but it's better with them as surprises for the viewer. That's not necessarily true of other Shyamalan projects - he fell in love with twists - but it's true of that work. 'Fight Club' isn't the same story if you know the twist - but it's still great. So is 'The Usual Suspects', or 'The Prestige', or 'Us', and on and on.
My humble twist is hardly at that level - 'Fight Club' is a completely different story with the twist vs without - but I do think it makes mystory more true to itself for the reader to find out what's happening (for certain, if they'd guessed) at the same time as the narrator finds out new information.