As most know, progressing through Planes of Power zones requires that certain tasks be performed. In many cases, one of the tasks required involves being on a raid that defeats a particularly powerful foe.
One side effect of this progress is that if the tasks were not accomplished in a particular order, a person on a raid may not gain the flag for completing the event, as they did not complete the prerequisite tasks.
The majority of "zone boss" style encounters in the first three tiers of Planes of Power zones have been changed to alleviate this problem of completing tasks out of order.
As of today, if you defeat one of these foes and do not have all of the prerequisites completed to receive the flag at the end of the event, you will be able to go back and complete the tasks that you have not yet performed.
Once you've done that, you may speak with the Seer in the Plane of Knowledge who will make sure that you are then able to progress as if you had performed the tasks in their proper order.