Seems like that arduous guy also wanted to avoid shiny codes. and he even made a list of pokemon that you can't get either because their pre-evolved form is not the correct type or they replace a type when they evolve
https://docs.google.com/spreadsheets/d/1kGUhsmwx6XgYKcBQrPLFUV7FkbSdWkHskp4rrMRws18/edit#gid=0