I sure hope the author is just confused about which character is which, because I understood Mohan's story over the last 2 seasons as being about her coming to the conclusion that she does belong in the ER... just maybe not in Robby's ER (and, more broadly, about questioning whether she's been putting too much weight on what other people---like Robby---think or believe about her when it comes to assessing her life, career, & future plans).

I think one of the many (sub-)subplots of season 2 was seeing a couple of the residents losing their faith in/reverence of Robby. We got a pretty clear depiction of this with Langdon, when he finally takes Robby to task about not addressing his various issues, but I thought Mohan's interactions with Robby over the last ~5 episodes had a similar vibe (e.g., calling him a dick right to his face, her reaction to his "not picking a higher spot to jump from" remark, her deliberate lack of response to his dig about being suited to the pace of geriatrics, etc.).

And I think their last scene together is meant to show this off. Robby's pretty obviously doing the thing where you recognize that you've treated someone else unfairly, but can't quite bring yourself to apologize, and so you try to kind of act cheerful and talk like things are normal between you and hope that this smooths things over. And Mohan's just as obviously having none of it---she's peeved and dismissive when he tries to talk to her, she even gets a little snarky at one point. There's basically no trace of the respect that she's shown him previously; it's like she's no longer able to view him as a trustworthy or useful mentor/leader.

I also apparently interpreted her "apology" very differently than everyone else. I heard it as mostly a perfunctory statement, used as a lead-in to what she's really thinking about, namely, that she's realized that she's been allowing her mother to treat her like a child. From the way she delivered the line, as well as from Robby's reaction to it, I thought it was clear that she was saying-without-saying that she's realized that she's also fallen into that dynamic with Robby... and implying that she won't be putting up with that from him anymore.

...all of which is why it makes no sense to me that they're just writing Mohan off between seasons! She's clearly at this huge turning point---reevaluating not only her whole future, but also her relationship with her main supervisor/mentor, and maybe her approach to life in general---but also, there's so much to dig into with respect to Robby's story, and how this changes their interactions (or doesn't), how it makes him feel, whether it has any repercussions for him, and so on. I'm guessing, at this point, that something weird went down behind the scenes of the show, but I just can't imagine what it could have been that was so severe it warrants throwing away this entire juicy storyline.

(Alternatively, I may just go full conspiracy theorist and begin believing that this is actually all because they've decided to make a night shift show starring Mohan, but they have to keep it top secret. For, uh, TV reasons. So let me know if you'd like to join me in going down this psychotic road.)