I don't think the cover says that this sequel is more about Director and Doyeon than Director and Yiyoung. I guess the author made it like this to make us believe believe that Doyeon it is a second ML who is interested in Yiyoung and a jealousy arc from Director .
Do they have to indicate it like indicate it to be obvious? This is a no brainer. Doyeon being at the fore front beside Kang means he has a bigger role in this series. Plus the title is also different. I would agree with your comment if the title says On and off: Work Love Balance. Means it's a direct sequel of the Kang and Yiyoung. In here there's 3 of them in the cover and Doyeon is one of the mains so his backstory and significant appearance in the story is OBVIOUS.
Idk why you all mad that this is mostly about Doyeon. I mean hello? the cover? It's Kang and Doyeon and Yiyoung is just in the background. Means this is more abt Kang and Doyeon and not Kang and Yiyoung. Also, this is a separate series/sequel right? So it is obvious that a different character is in the focus. I mean there's too much complaining instead of more using of logic.