pitch shifting of any kind (formant correcting or not) is meant to be used on monophonic sources, not on polyphonic ones. That's a common technology aspect, no matter which unit/brand you test. The pitch detector needs to understand which note is being input to the pitch shifter, than shifting is applied. A polyphonic feed would carry several different notes, with the result of "confusing" the pitch detector which is trying to shift everything with slight delays. Obvious artifacts happen in this case.
Sometimes when the source is a 2 voice signal, like intervals, carrying a 4th, 5th or octave your pitch shifter may be good at doing its job….