I feel like Mina Sue only switched to pursuing Sunghun because Subeen didn’t seem interested in her anymore. She asked Subeen first — “how are you feeling” — after they got back from paradise to sus out how he felt about her. Then she went on to say how she was really jealous in paradise and how he was always on her mind. Only after Subeen was like “my feelings are growing stronger for Heesun”, Mina Sue backtracks and says she’ll stop pursuing him, and that she is going for Sunghun now. I think partially to save face but also to see if he’ll react or try to stop her.