Fyodor is bored and lonely at the core - he itches for companion that actually can reflect back at him. He's eager for a worthy challenge - and he found that potential in Dazai. In that aspect Dazai refusing to join him and see things from his perspective (rejecting him) doesn't dispirit Fyodor, instead encourages. Easy yielding would turn him off, he loves a challenge. He'd be even more determined to break Dazai in whichever way possible, test him and attempt to shape into someone worthy of standing by his side (from Fyodor's perspective)

If it were just about Dazai shooting Fyodor's endeavors down Fyodor would have said something like "Dazai-kun is too blind to see world from my perspective. He had remarkable potential, truly a pity he turned out to be charmed by humanity's foolishness" - implies disappointment in Dazai's decision, but leaves the possibility for mending. If Dazai is brought 'onto the 'right' track' Fyodor would gladly see him as his equal again. However Fyodor stated Dazai is no longer apt for such role, hence implied disappointment in Dazai himself. That's why I think Fyodor expected Dazai to outwit him in Meursault or overall do something different and was left disappointed victory was so easy to attain.

Is Fyodor's current view on things necessarily objective and immutable? Of course not. Maybe not at the moment but later on Dazai can still do something to surprise him or even beat him for good. But for now on Fyodor is overtaken by disappointment and anger that he's truly alone on this world. Every human imperfection Dazai views as a virtue Fyodor does a vice. The fact Dazai tried to spin narrative around that Fyodor lost because of just that (reluctance to rely on human's unpredictability) probably left a sour taste in Fyodor's mouth, especially as Dazai himself was being duped in that moment. Of course, it might be just an affect, Fyodor himself ain't above emotions, however for the time being he's disgruntled Dazai didn't pass whatever test he had in mind for him