you ARE using the T-flight?
try going into the joystick settings in Windows - not in the game.
open the game controllers window in your thrustmaster folder -
click on properties - click on settings - click ‘reset to default’
do not calibrate!
close that and then check the rudder axis in the MSFS2020 in the ‘sensitivity’ window.
If the response to rudder input still shows unsteady or twitchy behavior your joystick twist rudder function is broken. At this point you can use the paddle for rudder control on the back of the throttle. Or you can punt and buy a new joystick.
Using the calibration tool was the problem on my end so resetting to default worked for me.