This is so fucking stupid. They started dating for real and then break up cause the "fake-marriage" plan is no longer necessary ????
uh, they never started dating for real. He asked her out and she eventually, after ghosting him for a while, agreed to one date explicitly designed to get him to leave her. They then started VERY platonically living together becuase of a work request they were essentially passively guilted into (it's Japan, everything at work is 'voluntary, not voluntary')
Even up to now, they're just two people who are convinced they're not right for romance who have a slowly building attraction.