Right, that's what makes it weird.

Kumiko is the one that feels guilty about the whole Middle School Competition thing and maybe even feels her words had hurt Reina, but Reina isn't affected by it. If anything, Reina might be confused why Kumiko is so standoffish.

All their relationship problems are Kumiko's issues.

The only mystery is why Reina came to Kitauji. Wanted a band that didn't take things so seriously so she could be the best? There's already a rivalry/jealousy dynamic building in the trumpet section. Likely about to come to a head during the audition phase. Is the beloved trumpet section leader not as nice as she acts? I am surprised to find that I can't wait to find out.