Mine does not react that way. If a car comes in front of me, less than the set distance, but not really close, and the car is matching my speed (or begins to slow) the ACC slows or brakes mildly.My major complaint with ACC is that it doesn't understand how to do anything subtly.
Car gets in front of you? OMG BRAKES BRAKES BRAKES WE'RE GONNA HIT THAT CAR AND DIE IN A FIRE BALL
Care gets out from in front of you? OMG I'M FREEEEEEEE! GAS GAS GAS GO GO GO GO GO NOW NOW NOW!!!!!!
I routinely find myself overriding the ACC just because here in the north east, we don't have a concept of safe following distances and people will just cut in front of you if you leave the smallest gap.