Thank you for uploading a version of your project.
Unfortunately, I am still not able to reproduce the behavior you describe. When I test with your project in FMOD Studio version 2.02.22, adjusting the global parameters does alter which return buses the signal is sent to: Setting the “Distance” parameter to any value above 1 prevents any audible signal from being sent to either of the return buses, and when “Distance” is at a lower value, changing the “RoomSize” parameter changes which return bus receives an audible signal.
It’s possible that this issue only affects certain version of FMOD Studio. Which version are you using?