It's funny how more and more groups are confused about who is sunbae/hoobae in industry

Confusing af.

But personally, I'll go over the group's debut and not the debut of one or couple of members to base the seniority if they're interacting as groups and not individually.

it's the safer option

If you use honorific and they are younger or newer than you no foul done

If you don't use it and they are older and more senior, you ass in on the line

Well, Ive has 2 members who are seniors, maybe that's why? And Yunjin is also not a new name in the industry, so make it even 3. Only Kazuha and Eunchae are rookies. So I kinda see why Gaeul would say that.

Also possible reason is they do not know which group debuted first, so to be safe just call the other one sunbaenim.