In Chopin Prelude Op. 28 No. 1 (C major) has a particularly mysterious mutated note in the 12th bar shown in the red box (this note does not repeat the voice of the previous blue box):
Is there a good reason why Chopin chooses to only mutate this note in the red box, not in the other bars?
If you look carefully, every bar has a right-hand melody with 5 notes. In each bar, you can see the fourth note always repeats the second note in the same bar; the only exception is what I highlighted in the red box and the blue box.