I thought they got mentioned a few times, at least by Akane herself, as working overseas or something like that, as partial explanation for her independence (whether or not they work far away, it's been implied that they're too busy to bother with their own daughter- reminder she had to get Ken to pretend to be her uncle for a parent-teacher meeting because of such implications). The other part of the explanation was reference to her late grandmother teaching her to take care of herself.
Looking back, they got mentioned at least once every chapter for the first four chapters, even if the last time didn't lead to any new details. And looks like Ch.6 had Akane first mention that they work overseas, so I did remember that correctly. Most likely brought up because Ken did keep prompting her about contacting them, at least with respect to the parent-teacher meeting from Ch.2. Ch.6 is also when her real grandma and her role in her upbringing came up.