Full auto switches rate of fire when ADS at some enemies.

When I ADS at an enemy with certain weapons. My weapon switches from fully auto fire to single fire, I never change the rate of fire to single fire and I have to push mouse trigger multiple times to get the gun to fire. It only happens when I aim at certain players during a match. I'm not fast enough to see what players. I don't have to switch fire rate after I've stopped aiming at certain players, it goes right back to full auto like nothing happened. I have survived some encounters. However, most end up in my death. I want to say it's "engineer" class more than most.