Honestly, I didn’t have any expectations that they would mention Aine in Ai’s episode, since that’s so much depth to go into to fit in one episode AND I predicted that they would be going by Debut since this season seems to focus a lot on senpai-kouhai relationships when it focused on Quartet Night.
Also note, in Debut, Aine wasn’t mentioned in Natsuki’s or Syo’s route concerning Ai. They dealt with discovering Ai was a robot and teaching him emotions while dealing with their own problems. Aine was only mentioned in charcter in Tokiya’s route, as “Reiji’s friend who disappeared.” So I think that’s pretty much the reason why he’s not mentioned in Ai’s episode.
I’m not keeping my hopes up that Aine will be mentioned in Reiji’s episode but eh, we’ll see.
At least they finally got to mention Ai’s a robot after TWO years.