I also noticed this, and then I scanned one cyclops, and later when we killed it, mission was completed. Like, by scanning you prove it's a cyclops.
I dunno which is worse, having to scan the the thing with the current xeno scanner implementation, or wasting the time to find and do the fight alone...You or somebody in your team (regardless of whether the mission is shared with them) has to get the final shot on an interceptor in order for it to count towards a mission.
This is how thargoid missions have always been.
I guess in the interest of the war effort, I will just quit this mission, stay away from this kind of missions, and focus on getting the bar moving.