It's interesting that this kind of throws a wrench into the obvious Arima-Miyahara plot. Everything so far had been set up as a way to show that despite his thoughts, Arima is really most interested in Miyahara.
But by introducing an (apparent to him) rival for Yukihira, that's a trope typically done for the actual love interest. So this is the first time the plot is setting something up to actually enhance his feelings for Yukihira instead.
Granted, we as the audience know this is a misunderstanding, but that is often enough the case with the trope anyway. They're keeping the story interesting.