FB - yes, the Airbus safety notice highlights also how the thrust from a fenstron doesn't change in a linear fashion with pedal position, it is more of an S shaped line on the graph. I think it has to do with the airflow around the duct which can provide additional anti-torque lift in some conditions.
I think this is why that kick in yaw you described earlier occurs when the SAS channel drops out at 40 kts - combined with the actuator effect I mentioned.