This video is from just a few minutes ago. Earlier I did the Bunker E15 lost sector, killed the three wyverns with precision damage and it didn't count at all. Unfortunately I don't have that on video but in this clip only two of the three counted for the triumph.
Thanks. From what I can tell, it did progress the triumph, since 1 of the 3 kills counted and you already had 1/3 prior to the start of the video.
My guess is there may have been some kind of perk bug or situational issue, such as only kills made outside of a lost sector or ones made during that particular boss fight don't count. There have been past issues with quests/triumphs involving lost sectors.
I suggest trying a Wyvern in a patrol area instead.