With what Yumi said, "why are you telling me this now?" it feels like she missed part of the conversation, like Sensei told her before something happened, not after, and Ayane and Maya, who are not part of "Sensei blackout", are aware of the "space time fuckery". It's pure conjecture tho.
From Sensei's perspective, he's having a single conversation with Yumi.
From Yumi's perspective, he's intiating a conversation, glitching again, resuming it later, only to glitch again, and again, and again -- always in private places, up until the beach, when he finishes in front of everyone.
"Why are you telling me this now" could be asking "why here, in public, where people who don't know about us can hear?"
Because there's clear intimacy occuring in the space between. In one of the stops, she has her uniform blazer on, and the bandages she uses to compress her chest, but no shirt -- and she doesn't appear distressed by the fact that he's there. That's indicative of a clear progression of their relationship.