1. First determine if the joystick cap is pulled up, or if the joystick physically does not return to centre. If so, pull the joystick cap off. If not, do joystick calibration (specific operation see the manual)
2. If it is connected to Steam, it may be triggered by a Steam bug (the current version of steam does have this bug), try to push the joystick to the outermost circle and rotate it for a fortnight to see if it can be fixed, if not, then rule out this factor.
Additional note: You can set the dead zone of the joystick of the Rainbow 2 Pro. When the joystick dead zone is set to 0, the joystick is bound to drift. Nothing to do with the product, all joysticks with 0 deadband will drift.