Maybe because, even if you bring your homemade lunch, you also join with others instead of eating your pack lunch since it's a team lunch.
I mean he's a newbie, who needs to interact with his seniors more, and packing a lunch to eat alone doesn't translate to that. It just mean you want to eat alone instead of hanging out with your colleagues.
But he just went and eat by himself all alone instead of joining others that made his hyung think he's being bullied.
Maybe it's a korea thing....

doing a reread since I’ve forgotten all about the first couple (as many of us seem to have done lol) and in chapter 13, everyone acts like it’s a huge deal that seonghan just…..brought lunch to work?? I just find it so confusing that dawun hears about it and his reaction is like “omg, making lunch for yourself while working full-time?? MADNESS, they must be bullying him!”….is there some kind of cultural context I’m not aware of as a westerner, or maybe something that didn’t translate well?