Because I don't want to spoil the whole thing in the comments, I'm just going to write it here:
This is a SLOWBURN. Of course readers would feel frustrated that Mia and Harry hadn't gone together - that's the point. Right now, Harry wants Cho, and Mia wanted Benoit, so naturally, their first choices for the ball wouldn’t be each other. In fact, Harry only asks Mia because he can’t go with Cho.
Yes, as a reader, Mia and Harry as partners would have been 100 times better. But this isn’t their moment yet. The tension comes from the fact that they almost could have had it and didn’t.
I also want to show Mia’s life outside of Harry. She isn’t a flat character whose only personality is orbiting him. She has her own friendships and relationships to build - in the Yule Ball’s case, that’s Milo.
And don’t worry - there’s still room for plenty of romantic tension. In fact, she’s already begun to regret not going with Harry, and that regret will only grow. ;)