Perhaps Mihiro is the first person who has really relied on Tsumugi, not just as a helper at the family business or such, so she feels good in his company. She has quite a relaxed if not unfettered personality, so she could interact with Mihiro and keep meeting him, despite there not being any deeper connection between them, and also despite the social distance between a working adult and a high school kid. I doubt she ever really thought about him as a man, though, before this moment when she saw his passion in his craft. If she had viewed him as a man from the beginning, it's possible she wouldn't even have kept in contact with him, due to that nowadays dubious social distance.