Same
I can understand how it can be a little confusing, basically, it helps to keep it mind that each omegaverse manga makes some rules of its own, it isn't really one consistent setting. Especially the interpretation of what fated pair actually is can be very different. This story treats fated pairs as purely physiological connection, special pheromonal compatibility. Before his first heat, Yuka is basically a pre-puberty kid, so he doesn't stimulate Taiga in any way. Only when Yuka's first heat comes Taiga goes into a rut. Kaoru, on the other hand, is in love with Yuka, so as soon as his alpha traits manifest themselves, ie. his body can go into rut, he's constantly well... turned on by him, even without Yuka actually emitting any pheromones. The main message of the story is that genuine love wins over bare physical predisposition, so it makes sense that Kaoru reacts to Yuka more strongly than Taiga.
I'm not sure how Taiga and Yuka were supposed to be a fated pair. In middle school, it showed Kaoru reacting to even just being near Yuka, but Taiga only reacted to him when Yuka went into heat. Before that, there was no indication of any reaction towards him. Kaoru, on the other hand, was sent into full blown rut just being within ten feet of Yuka. Taiga didn't even care until Yuka himself went into heat and drove everyone crazy. Maybe someone can explain it to me? I think I missed something.