My personal theory is that this is a "stable time loop." After growing up and learning that Seto fell in love with a man and got divorced, Nanasawa went back in time and took on the identity of Sasaki. Unbeknownst to her, Seto isn't interested in "Sasaki" either, likely due to being heterosexual, and had already met Sasaki before Nanasawa traveled back in time.
While Nanasawa met Sasaki in Chapter 3 and is thinking about her in Chapter 4, it's possible that Nanasawa forgot about Sasaki before going back in time, especially if Seto never gave any indication of being interested in other girls. Under this theory, it's also likely that Sasaki will end up learning things about Seto that Nanasawa never realized, such as why Seto never got together with Nanasawa.