Sorry that you've been having such a rough time getting this working. We had done some research ourselves a year or so ago about controlling an H9 with an aux switch and expression pedal, and we were surprised by how many cases expression pedals there are being sold where this simply wouldn't work correctly. My colleague, Nick, wrote a howto guide explaining how to set this up and he urged people to use one of the products we know to work for this. (We don't have any sort of financial relationship with the makers of those products by the way.)
Please take a look at Nick's howto guide. I think he mentions the problem you've been experiencing as something that can happen if one tries to use a volume pedal for the expression pedal together with an Aux switch:
https://www.eventideaudio.com/support/downloads/using-both-aux-switch-and-expression-pedal-h9
