For 2020 go to this path:
%LOCALAPPDATA%\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\Packages\inibuilds-aircraft-a350\work
Locate the ThrottleData.ini file
Ensure the following is set to True (the EFB doesn't seem to set it properly)
[throttle]
reverse_on_axis = true
This puzzled me for an hour or two initially but once I had changed it then the aircraft started observing the Idle and Max reverse detents on my Thrustmaster TCA Airbus throttle.