Responses
Haven'y read this one because I'm letting it marinate 1st. But if you're talking about omega having a mark here is Keith touching Yeonwoo's ear mark:
https://www.mangago.me/read-manga/kiss_me_liar_special_episodes_ssfujoshi/uu/br_chapter-126265/pg-7/

It’s the same world building as Kiss Me Liar, so why does he have a mark on his ear when he is an omega? If I remember correctly, omegas can have multiple alphas and alphas can only have one omega once marked.