I think Lunge is slowly realizing that Johan may actually exists. Let's cut the guy a break, it's hard for anyone to imagine that someone like Johan could exists. The evidence was pretty flimsy, just name records and a photo, and all the people arguing that Johan exists are people close to Tenma. The idea that Tenma has dual personalities made far more sense that some mystery shadow figure. For all Lunge knows, Tenma was just using the name Johan when commuting his crimes - that's how little evidence existed that Johan was a real person