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I've read on multiple study books (Bible, Manhattan, Trainer) that everything that is written on the stimulus should be looked as 100% correct and thus should not be questioned of its validity.

But look at PT13-S2-Q20.

It's a flaw question, and the stimulus is laid out in the following way:

"A can cause C due to reason Z.
What is true of A in this regard is also true for B.
Therefore, B can cause C due to the same reason"

The correct answer is A ("the argument treats a correlation that holds within individual categories as thereby holding across categories as well"). So basically, it's saying that the argument incorrectly views what is true for category A as equally applicable to the case of category B. But this correlation was already clearly established in the stimulus, so I don't see how this can be a flaw.

I looked through Manhattan forum but did not see a satisfying answer.

I don't think you represented the argument accurately. It's more like this:

P is more likely to cause cavities the longer it is on teeth.
Same deal with C.
C is on teeth for a shorter time.
Therefore C is less likely to cause cavities than P is.

Problem: Ignores the possibility that other attributes of C make it more likely than P to cause cavities despite its shorter time on teeth.

Those answer choices are tough though. If I had gotten that right, it would have been by process of elimination.