I am encountering the very same problem now with the RTM X-plane 11 with my Saitek Yoke and Cessna (a.k.a. "Madkat") Rudder pedals. Also using the yoke's throttle quadrant, a Saitek Cessna Trim Wheel, and a GoFlight TPM module, if that makes a difference. Fyi, I turned the control response and stability augmentation sliders all to the far left, and the turn coordinator ball does deflect somewhat, but (probably) not nearly as much as it would in real-life (at least not when compared to the A2A C172R in P3Dv3).
To me it appears that there is some setting that makes X-Plane "mouse yoke friendly" that needs to be disabled when a system with rudder controller(s) is/are detected. That, or expose the setting in the UI so that users can choose for themselves...