Connected a Boss expression pedal and aux switch to my H9. Had the problem with the pedal going from 0-100-0, or after adjusting the trim knob, 100-0 (the wrong way round). I then swapped the mono lead going from one end the Y splitter to the pedal with a stereo one, and all works well. So my setup is male stereo TSR Y lead to two mono female sockets. One mono socket connected to aux switch with mono cable, and the other mono socket connected to the Boss FV-300h (expression socket) with a stereo lead.
Hope this of help to some having problems.