I completed the raid with some clan guys and then during the final phase of the boss i begun to lag and my fire team said i was standing still, we then killed the boss but he was still alive and visible to me. i completed my weapon quest and got a helmet that dropped from the chest, but didn't get the milestone completed. i'm a little sad i wont have the milestone completed even after doing the raid before the reset but i thought it was interesting still.
-
all the milestone would have done is guaranteed that helmet which it most likely did its probably just a visual bug