Cpt
Hi, interesting point, and one we encountered in different circumstances: When we changed to the 'new software' DECU's last year, the same thing manifested itself (but engines not running, battery on - engineers check). Engineers went back to SK to try and resolve, but I left shortly after, before feedback). Good job you didn't try to continue the lever movement as the cable would have 'de-clutched' (as I think the term goes).
I don't think you can be encountering a 'sticky stepper', as even with the stepper frozen (ala manual reversion), lever movement would still occour, just moving the FMV.
I did have a passing fancy that maybe the resolver was freezing when trying to move the lever in mixed - this is the only thing I could think of that had some connection between the DECU/manual cable/FMV in mixed mode (though it's only supposed to monitor the FMV movement)
If the problem was with the cable/manual cable adapter then this would also be apparant in 'true manual' mode.
Out of interest, I think that FSI have changed their checklist for engine oscillations, to reflect the ease of diagnosis when you go straight to manual - ie freeze the stepper immediately - if you hit the wrong engine, simply freeze the other one instead! (this topic had some discussion earlier this year on PPRUNE).
Regards,
Doc