??? She doesn't treat Kang Bin better than Junghoon? Kang Bin is her employer and she has been explicitly told that it's 24/7 and she has to be available at all times. She's not neglecting Junghoon, she invited him out to dinner and is always trying to make room and time for him as best as she can. Even with this Hollywood thing, she really tried to persuade Kang Bin to let her stay and was sadden to miss her anniversary with Junghoon. I think you're being quite unfair in your statements.
Also, she literally can't explain it to him, as she is binded by a CONTRACT. That's like expecting a therapist to give away very personal information just because you're the boyfriend/girlfriend.
I can't help but instinctively think that malsook treats Kang bin a lot better than junghoon, whom she basically neglects. Junghoon is the perfect boyfriend, and I don't like malsook for hiding so much from him, not explaining or even trying to justify her actions, and basically not carrying her end of the relationship, leaving it up to junghoon to keep the relationship alive and happy. I still don't dislike malsook despite thinking all this, coz she is pretty cute with Kang bin. Junghoon should dump her and get together with his assistant who is in love with him