Over and over I see people complaining about how unpleasant Miya is. Honestly, I feel like her behavior is understandable. It's bad enough that her crush has a crush on someone else, but she's recognized from the beginning that he's a great guy. In fact, her first demonstrated memory of him is not only the moment he won her crush's heart but also demonstrated the exact sort of personality she herself admired so much in Asahi. Your crush is crushing on someone else? You're going to be boiling with jealousy. Your crush's crush is openly crushing on you and trying to get close to you? Add to the jealousy a strong helping of displeasure at being stuck with him constantly nearby. You recognize that, despite him not being your crush, he's got everything that made you like your crush? All the more confusion and discomfort. Put all of that stress in a girl who's apparently been subject to a fair amount of harassment in the past and has developed a defensive, reactive personality, and you're going to end up with someone very fast to respond negatively and very eager for an outlet to vent.
I do get the feeling that she's recognized from the start that she might have fallen for him if she'd known him first, which probably adds even more complexity and stress. I do feel like she's growing some degree of romantic feeling toward him, though she's not willing to admit it and probably has difficulty recognizing it so far since on the surface level there's no real change. Whether she's crushing on Sora or Asahi, the sight of the two of them together is going to trigger very similar feelings and reactions, making it harder to notice if it is happening. Personally, I can't help but compare the previous chapter to the pool-cleaning chapter in High School DxD, and Miya to Koneko. I recall Koneko being quite guarded and uneasy as Issei turned to her when she came out in her swimsuit, and her confused emotions when he didn't respond with any sexual desire toward her. Practically the same thing happened here. She was quite guarded and uneasy about how Sora would react to seeing her in a swimsuit, and clearly felt quite conflicted when his response was just "cool".