Hey murambi,

I dont know whether it will help identify the problem but you can try. Under show option uncheck polygons, nurbs and subdiv surfaces. Keep the joints, deformers, nurbs curves and locators checked.

Now click on any of the joint or controls near the face where the deformation is happening. Note down its rotation values . Rotate the global control in y axis now . Click on same joint or control and check if there are any changes in the values.You will have to do this on every control or joint on the face.

Again i am sure of it but there is no harm in trying.

Regards,
Mahesh