Thanks, I finally understood the issue. Cue is supposed to make the processed channel audible, that means including all EQ and Dynamics. As such the Cue signal is taken right at the output, and will indeed include Talkback most of the times. We have this design for 15 years or so, and everyone else seems to not have an issue with that - we never got a report.
Some units allow to Cue without Talkback, but only when EQ is disabled in both Mains and the Cue channel, like UFX+, UFX II and UFX III. Your old FF400 and 800 did not have any (output) FX, there excluding Talkback was easy and not coupled to complicated DSP resource management.
That said a design that allows a separate handling of Talkback while Cueing is rather complicate in hardware and will most probably not come. We might prevent the simultaneous use of Talkback and Cue in upcoming versions of TM FX to make clear how it works.
Regards
Matthias Carstens
RME