Your interpretation of Jamie indicates that you didn't really get to know her from playing the AVN.
"least ambitious" - LOL, that cannot be taken seriously.
"most dedicated to the MC" - compared to what/who? MC is a closed-off loner with intimacy issues. He ran through a string of girlfriends and the only constant friends in his life are those that have known him since before he went to law school (Luis, the Carringtons, Sean). The MC still keeps these friends at arms length emotionally.
"decides to exit stage left while he's going through a hard time because of her sudden ambition" - you really seem to have missed a lot in this AVN regarding both Jamie's ambitious career goals and her relationship with the MC.
As a primer, replay the scene in Ep 3 where Jamie meets Seo Jun for coffee:
Rockford had a year (working close to Jamie) to sort his life out, sort his feelings out, and work out how he really felt about Jamie - the adult. He mostly failed.
Jamie came to the conclusion that Rockford may never see her beyond her being "Trevor's kid sister," and accepted she might have to move on from her childhood crush.
Jamie gets a once-in-a-lifetime job offer that aligns with her previously stated career goals and makes a decision to move on.
A day after Jamie makes her decision, Rockford professes his love for her and puts her on the spot. Jamie is extremely upset over her own decision, not because she won't have a romantic relationship with her crush, but because she knows she's abandoning a good friend in his time of need. Nevertheless, she also has her own life and career to look after.
If and when Jamie has a change of heart, it will be a natural progression, not a soap opera scene in Ch 4, where she immediately changes her mind and beds Rockford.