If you speak to Rebbes today and ask them the question as to how was this possible (that gedolim could not see the truth), they will tell you that Hakodesh Boruch Hu took away the capacity of Gedolim to know and see the truth. It was a “hester ponim” for which we have no answer.
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The hester panim explanation resolves nothing. Why should an average yid seek the advice of a gadol on any issues if there is the possibility that G-d is “mastir panav,” and therefore the gadol has a distorted view of the world? More poignantly, since G-d knows that Jews will seek the guidance of gedolim, perhaps He skews their understanding and not that of more pedestrian people so as to achieve His goals? Might that be p’shat in the ma’amar Chazal that bizman hazeh prophecy is the purview of lunatics and children? They see clearly when G-d blurs the sight of the gedolim?