Problem with Joby S4 throttle?

I'm using a Thrustmaster stick with built-in slider for throttle.  It works fine with planes, but for some reason, it's acting weird with the Joby.  No matter where I position the slider, it snaps back to the middle by itself.  As far as I can tell, I don't have the throttle slider bound to any other controls.  Anyone else run into this issue?  I saw a post about it on Facebook, but there were no responses.