Does it happen more often during extension or retraction?
The question is does flap asymmetry occur. My answer is that in 40 years on airliners I have never seen flap asymmetry. I have seen plenty of times where the flap asymmetry detection system operates and shuts down the flap motor and applies the brakes and puts up a flap asym message, but it has always been indication to me. I can recall one slat asymmetry where aa gearbox sheared on a B767, but no flaps.
The flap asym brake system is so good on airliners because the effects of real flap asymmetry are so great.