So, basically I did this encounter two times with a full fireteam of arc guardians and it didn't count. I was playing with randoms on an LFG group so I cannot trust 100% that EVERYONE was using arc but I'm pretty sure they were. Second time ppl changed to arc before the encounter ended but first time we were all on arc all the time. So I discard the "you cannot change at last second" theory. I got it on all the other encounters. I don't know if this is a bug. Did someone have this same problem? I'm on PC btw. Thank you! I hope Bungie fixes this or at least gives clarifications on the subject.