I'm starting to think that maybe Akagi has memories (or is) from an alternate reality or something? The Knapsack Mai previously appeared in the movie as something with the power to travel between realities, so her reappearing even briefly this season makes me assume something has happened and someone here is not from this reality, right? We know for a fact that the kid version of Mai is tied to the desire to escape reality. Then we get the reveal that Akagi remembers Sakuta writing something in middle school that he didn't actually write, as he'd yet to meet Shoko.
But Akagi does fit pretty naturally into the story so far, she doesn't entirely feel like she isn't from this world. That being said, in that alternate reality she also went to Sakuta's highschool and had a much closer relationship with him. There 100% is precedent for her specifically acting differently in the other world. I'm pretty confident that she's not from this reality, but I just can't tell behind that.