We checked our 9C. We didn't find this difficulty. Pmix 6 and 7, have no TRIM function, but all mixes have the LINK function. This is with the transmitter set in ACRO mode.

We mixed Rudder to Aux 1, and then made a mix with Throttle to Rudder. We used Pmix1 and Pmix2, turned trim ON both and Link on the rudder to Aux1 mix. Rudder operated both channels, and when we moved throttle, both channels moved. The rudder trim moved both rudder channels, and the throttle trim moved the throttle and both rudder channels.