Oohhh which group are you talking about?
B.A.P!
Daehyun and Youngjae.
I don't know if you heard of them before, their story is quite sad because they were under the shitest label.
B.A.P quick started the "bad boy" concept with progressive lyrics that question and expose society's issues, and paved the way for the groups that came after, including BTS.
They went their separate ways after the contracts expired in 2018/2019, but the members are really close and kept baiting us with a comeback that finally came this year.
The members I talked about are Daehyun and Youngjae, the mischievous duo.
Youngjae never called Daehyun hyung, they were super close and we're always teasing one another. DaeJae is most fans OTP. (I'm more into DaeUp tho)
I can imagine Daehyun's dumbfounded expression if Youngjae actually called him hyung after 12 years, it would be something like this:
YJ: Daehyunie hyung
DH:............... What the fu*** did you call me? щ(゜ロ゜щ) Are you alright in the head? Did they feed you something weird in the military? Have you been in an accident and didn't tell us? *Nervous laugh* Do ya want something from me? *Let the satori accent slip while starting to panic* ANSWER ME!! ヘ（。□°）ヘ
YG + JU: *laughing until the brink of death*
It reminds me of the only K-Pop group I like.
There's two member that are 1 year apart from each other and the younger one calls the older in an endearing way but never called him hyung since before their debut in 2012.
I think if he did it one day, it would be super awkward but also touching.