When slowing and setting speed on autopilot the throttle shoots up and down quickly and causes the plane to start bobbing.
(The bobbing issue has been raised many, many times but doesn’t seem to be being fixed)
Other airplanes have a much smoother autothrottle, such as the 737.
When you see pilots in real life setting speed on the control unit the throttle reduces gently and causes a gradual bleed off of speed. Flaps are then deployed at the appropriate speed intervals and the aircraft may pitch up slightly due to the increase in lift but then settles into a stable pitch.
I really think a more realistic representation of the aircraft physics at the approach and slowing down phase of the flight is a priority on most airplanes in IF.