Inconsistent ATC start-up / taxi permission behaviour

[Pizzicato]

I'm putting together a Cold Start "refresher" tutorial mission for the Hornet, but I'm experiencing some weird ATC behaviour that's resulting in an inconsistent tutorial flow.

Basically, there are some occasions when starting up the Right Engine will immediately result in an automated "Ready to Taxi" message (just the text, no voice) from my virtual pilot. On other occasions, I have to actively communicate "Request Start-Up" before the "Request Taxi" option becomes available. 

Does anyone know what triggers the automated transition from "Request Start-Up" to "Request Taxi" ATC options?