My sense of it is that Kaibara still wants to "train" Yamaoka further in the art of food, for the sake of "making the food culture better". Like, in the cases where Yamaoka loses, for all that Kaibara berates him, he still explains where he went wrong. And in other times Kaibara drops hints and allows Yamaoka to figure it out on his own.
Letting him win is a kind of carrot and stick, I suppose, because despite everything Kaibara seems to acknowledge Yamaoka's potential and ability. Yamaoka might ragequit cooking entirely if Kaibara had just stomped all over him every single time, and that'd be kind of a detriment in what Kaibara is trying to do.