Aircraft have inertia.
If a "headwind with updraught" gust hits the aircraft, it will momentarily give an increase in IAS and in lift, which will tend to amplify the updraught effect because the wings will generate an addition to the upwards "bump".
If the same aircraft is travelling downwind, the same gust will be "tailwind with updraught". The reduction in IAS will decrease the lift, which will offset the "updraught" and give a lesser "bump" effect.
Obviously, this is one case where our oft mentioned "aircraft on a conveyor belt" might be an easier ride....until it takes off, of course.