My first thought: did you remove www/build from your .gitignore file? If not, that would make sense why it’s having trouble with www on the server, but not locally - because git is not tracking the built ionic app and thus not pushing it to Heroku.
If you already did that - Do you have your code in a github repo where we could look at it?
Note that if you were creating a real mobile app that you’d deploy with cordova to an app store, you normally wouldn’t include www when hosting the backend on heroku, but because this is a simplified web-only tutorial, we are pushing up the built ionic app so you can view it in a browser!