It seems unfair that Baba appeared in a disguise and then blamed his devotee, Dev, for not recognizing him. How could Dev possibly know the Sanyasi was Baba, especially since that Sanyasi had previously come for a different reason?

Your question touches upon the heart of Dev's own confusion. In Chapter 40, Dev himself reflects on this exact point, admitting his vision was clouded. He notes that the Sanyasi had first visited to collect funds for cow protection even before the invitation for the Udyapan was sent to Sai. This prior context made it difficult for him to infer that the Sanyasi asking for a meal was Baba in disguise. However, Baba's lesson seems to be about faith and paying close attention to his words. He had specifically told Dev he would come with two others, a detail Dev later admitted he "forgot to pay attention to." Baba's response to Jog, "Why then did you call Me? You did not recognize Me!", suggests that inviting him requires a level of awareness that transcends physical appearances.