Nothing is wrong with the art. That’s an example of if they draw all the characters with same hair color. You can still tell the difference if you focus enough. you have to recognize the characters’ faces or something that is stand out about them.
It was easy to recognize before the hiatus, but after the hiatus/season change, it got worse. The art itself has gotten better, but the characters’ faces and body types all look the same. Apart from the CEO and his assistant or whatever, I can barely tell them apart not even Sejin anymore.
Sejin gets taller and looks more manly so that’s why you can’t tell the difference between him and Seju. As for other characters, I also have a hard time telling the difference but if you focus enough, you could see the difference between them. You can try to tell the difference by remembering how they dress and personality
THE ART!! THE ART WHAT'S WRONG WITH THE ART!!! I LOVE THE PLOT BUT THE ART IS MESSING UP WITH MY MIND