Handling toast notification in Windows Phone 8.1 when app is closed
I do not know technical reason why it is not working as you have implemented it right way. But there are two points I want you to check and I hope your issue will be solved.
Try using pushnotificationreceived event, it will give you wholetoast notification xml string and you can further work on it.
It is possible that the page is not able to load the whole html and javascript when a launch is triggered. So you can use Window.SetTimeout for 1 second or more seconds delay inside the If condition of activated event handler.