I also caught that. It's odd. And tbh, I feel like this story was lacking a bit in terms of highlighting the relationship between Naoki and Chizu. They don't feel very close. He doesn't really hug her or kiss her or Pat her, basically no physical contact except the minimum v.v I just find it interesting how she calls her mom mom but her dad Noaki as well. I really like the story alot but it feels like it we missing too much important emotional elements that give the characters more depth. Like strengthening the relationship between Noaki and Chizu, and maybe getting chizu to say how she really feels about Noaki and how she wanted to stay or that she think he doesn't want her. But they didn't address any of that. So it's like meh. But that's just me.
Wait...so why did Chizu call her dad "Naoki-kun" the whole time instead of "Dad"? Is this not unusual in Japan?