I've noticed this same problem with my Saitek x52 (non-pro).
I had Z- mapped to "Collective Increase (Analog)" and Z+ to "Collective Decrease (Analog)". While this seems strange, for whatever reason Saitek designed this stick to be at a value of 255/255 when in the 0% throttle position and 0/255 when at the 100% throttle position.
While in this configuration, 50 percent was neutral (gravity caused decrease in altitude), 25% started decrease in collective, and 75% started increase in collective. From 25% throttle to 75% throttle it was neutral and no increase or decrease in collective was observed (gravity naturally caused slight descent).
I applied the fix above to map both Z- and Z+ to "Collective Increase (Analog)" and found this to function as expected with 0% being 0% throttle applied, 50% throttle being 50% throttle applied, and 100% being 100% throttle applied. However at first configuration, something strange happened.
When you map Z+ above Z- in the keybinds for "Collective Increase (Analog)", the throttle behavior is reversed (0% throttle is 100% throttle being applied and vice-versa). When I switched the two keybind positions (Z- being atop), the throttle functioned as expected. I'm not sure why this is. If anything, I would have expected that there was throttle applied at 100% all the time or that a deadspot would be present around 50%.
Thanks for the workaround, definitely upvoted.
Just thought I'd provide some food for thought...