Rails+Heroku does not deploy package.json dependencies

We are developing the Rails project by deploying it to heroku.
I was able to work on package.json in the dev environment as follows.

{
  "name": "app-rails",
  "private"—true,
  "dependencies": {
    "primer-css": "^9.6.0"
  }
}

After deploying it, File to import not found or unreliable: primer-cs/index.scss..
herokurun cat package.json did not reflect dependencies

{
  "name": "app-rails",
  "private"—true,
  "dependencies": {}
}

Is there a lack of setting that does not reflect dependencies?
Thank you for your cooperation.