Issue
Apps containing git submodules do not deploy correctly when using GitHub based deploy methods.
Resolution
Unfortunately GitHub repos that use submodules will generally not deploy correctly on Heroku. This is because GitHub does not include submodule contents when repo-content tarballs are generated.
For more information, please view our Dev Center article here:
https://devcenter.heroku.com/articles/github-integration-review-apps#do-git-submodules-work