Yes. He would actually be surprised by Faustine's loyalty, because she's concealed how much she likes him.

You could easily turn the question around. Why did Harry obsess over Draco when he understood him so well? The main problem is that Harry was prepared for the possibility that Draco would betray him (although not enough), while Draco was utterly shocked by the notion that Harry meant more to him than he'd originally supposed.

And thank you!