There isn't supposed to be, I don't think. Distortion field breaks missile locks once, and that's already pretty powerful.
As with any other lock break, breaking the lock before the missile is actually launched means the attacker's missile is not on cooldown and thus he can immediately re-paint the lock.
You might also be falling victim to the bugged double volley cluster missiles, which require two separate lock breaks (one for each damage packet) to fully prevent.
Of course, you could argue that since engine abilities prevent locks for the duration of the maneuver (which I believe is true, though I'm not positive), distortion field should also prevent locks for a duration. Obviously a six second immunity to missile locks would be grossly overpowered, and I'd argue that even a three second immunity to missile locks would be overpowered. Furthermore, since most maneuvers bring the victim out of the attacker's missile range and/or firing arc, I think there is little practical difference between engine abilities preventing new missile locks from applying and a theoretical change that would simply break current missile locks without this prevention.
It would have been more balanced when Distortion Field's active was 3s.
Like I said though as a missile break it's.. kind of dubious.. when I wait for the missile to fire, I hit distortion field still get hit, every time.
Can't be all upgraded cluster missiles.