I seen that in the last patch/mini update? You made that achieving a flawless victory in the throne realm will get you the "defeat [boss name] flawlessly" achievement. And I realized I don't have the "kill the assassin with the bloom effect" achievement unlocked, so I proceeded to kill her and in my first try I was able to defeat her with the bloom effect being my final blow. But when I went to see if I had unlocked the achievement, I realized that it is still locked, so I tried again, and it didn't work. Hope this gets fixed soon! (Or maybe it's intentional?)