Facial Animation inconsistancy in Mecanim

I have a character using the Mecanim Humanoid system with Extra bones for the facial animations.

When the time comes to play my animations, I get really weird results; The jaw bone (which is mapped in the avatar) either animates in reverse or parts of the skinned mesh seam to lose some of its influence to the jaw ( ex: the chin will go down, but the teeth will only move slightly even though they are skinned 100% to the jaw)

If I go in my characters's Avatar and remove the jaw bone from the Mecanim mapping, one of my animations will actually start working : the jaw will move, even though there is no reason it should move at all! but only for that animation. all the others will disregard the jaw animation.

I doubt it has anything to do with my animations because I can put my system in legacy and everything works like a charm. More so, I am currently working on two characters with the same system (Mecanim and facial rig) and that one works fine!

Thank you for your help!