Why don't git submodules work with Heroku Pipelines, Review apps or GitHub Sync?
Apps containing git submodules do not deploy correctly when using GitHub based deploy methods.
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. https://devcenter.heroku.com/articles/github-integration-review-apps#do-git-submodules-work