Very mysterious. I would suspect your Y-lead. Try using the one we recommend.
I don't think that's the problem. We tested the Moog pedal when we were putting together this document about using an H9 with an expression pedal and aux switch:
We saw the same thing. If you want to use an expression pedal and aux switch, you really should use one of the expression pedals that we recommended in that document and know work. A lot of the expression pedals we tested didn't work in this configuration.