Looking for open source pitch modification algorithm with formant preservation?

For the past few weeks, I’ve been trying to find an open source algorithm which can be used to change the pitch of a vocal recording, similar to autotune.

I have found many open source libraries and examples which are capable of changing the pitch of a recording, but I cannot find any which don’t affect the formants.

I tried the suggestions for a similar question, but I have issues with SoundTouch and Rubberband.

Rubberband’s formant preservation option is not correct when compared to other plugins, such as GForm, Waves Tune or Melodyne.

SoundTouch, jungle.js and Tone.js all seem to distort the formants. I am not certain about Autotalent, which I am testing now.

I’m trying to find any piece of code which doesn’t affect the formants, or if possible, code which allows the formants to be modified separately to the pitch.

Any suggestions would be greatly appreciated!