Haewon didn't care about rumors about him but to him, it's different when Yun-oh is getting the brunt of it especially when it's reaching school. He's a very protective hyung. Yun-oh is a freshman with confidence issues. He has a few more years left in college and the kinda rumors that's floating around is gonna get him isolated and nasty looks until he graduates.
Yun-oh already has a complex on how dependent he is on Haewon despite not having much. If he gets wind of rumors defiling's Haewon's reputation due to their relationship, he'd feel like he does brings nothing to the table but trouble and might sacrifice the relationship to protect Haewon.
This manhwa delves into the drama of age gaps. Nasty rumors is inevitably a part of it
Complaining about an angsty drama manhwa having drama
"Assure your partner and find a solution" when both of them have history of keeping shit to themselves because they don't want to trouble the other person. If anything, this arc would be the opportunity to show if they actually learned the lesson of transparency and communication from the previous arc.
And it's not as if the drama is irrelevant. It's an expected outcome from their weird dynamic, and relationships with colleagues and family.

These rumors are such unnecessary drama. ATP it’s just men ego that are getting hurt.
So what if ppl spread rumors? They know it’s not true anyways and as long as he doesn’t get bullied for it.