If Ian treats her well, she would have no reason to leave him, in contrast to Wade. You can totally be disloyal to one person and loyal to the next person, since these persons and circumstances can differ hugely. Of course, the habit of keeping to cheat is a thing and can happen, but it doesn't has to.
You are laser focused on Ian getting cucked by Axel. It is a scenario that can happen in one way or another, since why else bring back the toxic scary ex, its a staple in porn and hentai. But ORS main feature are the many routes and possibilities, so wholesome vanilla happy end routes will also happen. Eva won't needlessly scare away half their audience just to satisfy another half. Not when the game is designed to have many routes and variations to please all sides.