project.reload_mesh - Callback fails when run on startup? - returns instantaneously.

Hello, I have a script that is run on startup, it loads a project file, then is supposed to reload the mesh etc

The reloading of the mesh is done by calling a seperate script, and via project.reload_mesh(path,settings,callback)

I am using the callback to delay proceeding with the script until it has been rebaked fully.

When I load this script directly via the python drop down, this callback works flawlessly - however, when I call the same script via my startup scipt, the callback is instantaneous, and does not wait for the bake to finish. 

Is there a reason for the difference in behaviour? And is there a workaround for this please? 

Thank you! 
Graeme