Hi ! 

When i rig a character i have

Tronc->cuisse->mollet

If i move "Cuisse" (Y -3 pixel) and set a key, the "mollet" move too (Y -3 pixel) , but if i try to set "mollet" to (Y +3pixel) normaly it doesnt move.. but there is a shaggy thing like right left pixel movement.

(Ps: there is a possibility to set a key for every existing option ?)

Hey, can I have an example project of the issue if possible ? 

Is it something simlar ?  Parent go X+5, child X-5 and the child doesn't move, but maybe in your case there is rotation.