Responses
They answered this in the QnA, kyusung wasn't dating anybody after he broke up with jooha. Bcs of the accident, jooha felt depressed and had low self esteem plus kyusung got busier with acting, and he got the wrong idea and thought kyusung was with someone else then broke up with him.
The writer explained they didn't include kyusung and jooha story because they just want to focus on the main couple story.
If they had to explain the plot in an interview then they failed as a writer (at least with this specific manga).
The whole plot is very bad, the misunderstanding felt artificial and forced. Also, who was Kyusung dating then? He broke up with Jooha, clearly he wasn't with his cousin so who was it then? Very strange story.