Have you checked on the servo monitor. It maybe possible that the aileron stick is not correctly calibrated, and has an offset that is then applied to the rudder when the mixer is activated.
With the trim centered, the servo monitor should show no aileron deflection.
Also check on the servo calibrate screens that the aileron control line is in the center position.
If the fault is only on one model make sure that the mixer switch is not allocated to another function, i.e. if it changed flight phase then this could also change the rudder trim position.
