Humanoid avatar mask impeding layered animation

I'm trying to make my character perform the same attack while crouched or standing with an avatar mask, but if only the legs are selected in the mask, the character's legs retract up and the character hovers for the duration of the attack animation. If the shadow is also selected in the avatar mask, which I believe is the correct way to do this, the torso lowers but the animation seems to twist around the chest joint instead of the hips. As best as I can tell, the shadow in the avatar mask is the root node, which Unity assumes to be the hips in a humanoid rig, but my character behaves as if the chest is the root node. Scant documentation exists in this area and I'm at a loss for why my animations behave this way.
Animations do not use root motion and both layers have the identical crouch/idle blend trees, but the torso layer has a transition to an attack state and back. I can make a generic rig work, but I would prefer to use a humanoid rig.

 

Everything should work exactly the same for humanoid and generic rigs, so if you can get generic working then humanoid should too. If it's not, you should report a bug.