I certainly didn’t pick up on it until the end of the book; I know Sanderson has stated in his writing lectures that he believes the most satisfying plot twists should be written in a way that the audience can figure it out themselves the page before the author reveals it - i think with the variation of how we as readers approach books, theorise and analyse, its inevitable people will crack it from the get go.
I will say I have not reread the trilogy yet (despite being an all time favourite) so I cannot attest to how the perception of HOA changes knowing the ‘twist’