I had the same issue, in the responses of the support they insisted that I had done serpent's den and thus wasn't eligible for the reward of the event, despite it clearly stating in the patch notes that the twisted serpent stronghold is the raid in question for the event and since it's the easier one to do, that's the one I managed during the event. With screenshots of the ingame achievements showing that I haven't killed any bosses in serpent's den the support granted me the reward for the event, yet I think it's pretty obvious that there is some mislabeling and confusion with the names was going on. Maybe someone @Hime someone could look into that, so that there won't be any confusion in case other events concerning that raid happen in the future.