@Trog my interpretation is that it was less about Nasa actually forgetting something important but more the fact that it reminded Tsukasa that even someone as amazing and someone that loves her as much as Nasa is human and her fear of losing provoked her to get some distance
Chitose simply...
Basically a case of both parties assuming without communicating and when they "give up" it only feeds the confirmation bias of the other.
Communication is key.
Yooo thanks for explaining it @sarusa
I've read enough manga to know about it but it was great to see the confirmation and explicit examples in the chapter