Invisible C0 at start

I am trying to use a guitar patch set to one of the GM guitars in Halion because I need a lower range (F1) than any of the non-GM guitars support (think they are limited to A1). I also prefer to work with GM instruments for clarity.
However there seems to be an invisible C0 I can see being triggered in Halion upon initial playback. This is present in a new blank score and audible as soon as I change to a GM guitar. It is also sustained, so renders playback unusable.
I’m guessing it’s some kind of keyswitch or control message, but I can’t see it anywhere in Dorico. How do I fix this ?

I’ve quickly found a workaround by changing the channel (and on Halion) away from Ch1 - but still curious about this

Indeed, you’re quite right: if you choose a different sound in HALion Sonic SE, unfortunately Dorico doesn’t get any kind of callback from the plug-in to let it know that you’ve done so. As such, the expression map that was automatically allocated for the sound that Dorico chose for you remains in force, and any keyswitches etc. that are part of that expression map will still be output. You can choose the ‘Default’ expression map or another appropriate one from the Endpoint Setup dialog.