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(The answer to this actually comes form user Kenneth Reitz, which was answered on the Heroku support channel. Thanks!)
Since this didn’t appear to be a problem related to the application code, the Heroku team suggested I purge the build cache, using this tool: https://github.com/heroku/heroku-repo
The commands I ran were:
heroku plugins:install heroku-repo
heroku repo:purge_cache -a project-name
That seemed to do the trick!
Source:stackexchange.com