I was able to replicate this issue on basically any "rescue" type mission. When rescuing a person in a group of enemies, if the enemies are using AOE dmg powers on you and you are standing near the NPC, the NPC takes damage and eventually dies. This makes it appear as though the person who saved, as the objective does drop 1, but the expected behavior should instead be an outright Mission Fail, as otherwise when you rescue another NPC, the objectives go away, and then you are stuck in the mission without being able to complete. In the case of Ouroboros missions, leaving the task force and restarting is an option, but for a character leveling up this may not be the case.