I probably wouldn't use the external cause code or any additional ICD to the nonunion fracture one in this scenario since the patient seems to be seeking such late treatment, from what you describe. Sounds like she inured herself 5 months ago, the fracture healed incorrectly whether she sought treatment or not initially, and now the provider is treating the non or malunion.
You don't necessarily have to use an external cause code with a fracture every time. While that may be the "classroom correct coding way" in this case, unless the provider specifically stated it and documented it, I would just leave it off. That's probably the reason you are seeing the "missing invalid diagnosis" denial or rejection. Also, read through your ICD-10 guidelines, there is a little sentence in the external cause coding guidelines that states, "assignment of the 7th character for external cause should match the 7th character of the code assigned for the associated injury or condition for the encounter." So, that could also be why the edit is hitting. It's better to just leave that off. Even though in this case, the V code does not have the K last character option. Also, think of it this way, the falling from the bus is not what caused the malunion or nonunion, it is what caused the initial fracture 5-6 months ago, so technically, that's not exactly the case for the nonunion. The nonunion is that the patient didn't get treatment initially and it did not heal correctly. That, or they could have DM or some other reason why their bones don't heal correctly. I guess there could be a case where someone had a fracture, it healed and then they re-fracture the same spot due to falling, but that's not the same thing as a nonunion, that's more a "new" fracture.
Most places in a pro-fee outpatient type setting don't add the external cause or the place of occurrence unless it is the very first visit for a new injury. Even then, I have seen where it is only added to an ED or urgent care type claim. Inpatient and facility would be a different story.