Skull Slugs question

I've completed the worst part of the mission getting the special shotgun headshot kills on operators over 12 meters, but I'm unclear why I haven't gotten as much progress on the 2nd requirement. It just says to kill 12 operators at a range of over 12 meters, which I've done 8 times at least to complete the 3rd requirement. Is there an unstated condition for the 2nd part? I know I've gotten other operator kills in excess of that range but it's not counting toward progress. I'm at 4 of 12 right now for general operator kills at over 12 meters.