There are always interactions between roll inputs and yaw inputs.
In order to stay in one spot the main rotor has to provide a side force equal to the force the tail makes to counter the main rotor torque.
This always causes the heli to fly slightly tilted to the right, and means when you change the tail rotor speed to yaw the lateral forces are no longer balanced as well.
This is not just for the MCPx, but is true for all helis. (Except If the rotation of the main rotor is reversed then the lean will be the other way