Bug Configurable joint stops moving with animation controller

Hi!
I've got a character which moves with root motion enabled through my level, and I've set it up so that the animation has keyframes only in the base and spine bones. I've put a configurable joint in the head bones so that it moves as a ragdoll when the character moves around.
The thing is that the animation freezes and overrides the joint even though it doesn't have keyframes there.
I've tried "Bake into pose" in all configurations possible, also tried using an empty animation and the joint still freezes, also tried no root motion, still the same. Also tried putting animator in "animate with physics" and still nothing.

One strange thing is that if I tick and untick "write defaults" inside the state the character is in the animator controller, the joint unfreezes! but that just seems to work during play.

Any help would be apreciated, even if it is a recomendation of other way to implement a spring between 2 bones that works with an animator.

Thanks in advance.