Can you explain how Dev's own past experiences and assumptions led him to completely miss that the Sanyasi was Baba in disguise?

Dev's failure to recognize Baba was caused by a 'veil of Maya' strengthened by his own assumptions. According to Chapter 40, Dev had encountered this specific Sanyasi before he had even invited Baba to the Udyapan. During that first meeting, the Sanyasi was collecting funds for cow protection. Because of this prior context, when the Sanyasi appeared for the meal, Dev simply saw him as a guest who had come for his own purposes, not as a manifestation of Baba. Dev later lamented his own foolishness, realizing that this prior encounter created a 'delusion' in his mind, preventing him from connecting the Sanyasi's arrival with Baba's promise to attend with two others.