I was able to get around it by pushing all files to heroku git instead of mentioned files in documentations
So, instead of,
git add -f Procfile config.json package.json build && git commit -am "adding Procfile and configs for Heroku"
git add . && git commit -am "adding Procfile and configs for Heroku"
This solved my problem.
However, I have moved away from Heroku and deployed my forum on GCP.