If there wasn't the one scene in the beginning, where you see her standing(with a shadow) with the army in the first panel. I'd of assumed that the FL is actually dead by the time the King completes his goal. It just seems that way with based on what I know now(up to chp 41).
1. You don't really see her kneel, when the King lowers his hand.
- Obviously, it could be a normal "zoomed in" scene to show what affect the authors want to get across.
2. She takes her duties as a knight super serious to the point where no-one would ever question her. There is absolutely no way that she wouldn't of been the first to of knelt down, nor would she ever stand up without his command.
-Obviously, the authors could of just meshed the "ending" in the beginning to get the story going. So, if you think about it like a time-skip to when he lowers his hand to when the FL is behind him in the next scene it would make sense.
3. The moment he turns around and sees her you could look at as him being shocked that he sees her as she died before he accomplished his goal... So, it was like a final farewell from her.. "I have stayed next to you as your knight until you accomplished your dream."
-Obviously, its probably exactly what it is. The King accomplished his goal and came to realize he's fallen in love.
NOTE: This is just my opinion.. So, being picky with the details I just wanted to say how it could easily take a different route just from a few panels or it could just be exactly what it is.. A king falls in love at the end of his conquest(which is most likely the case).