Today I had a lesson in RTFM. I noticed my Hugo public/ folder had a bunch of orphaned content from a previous deploy that used dummy content. There is a clear reference to this in the Hugo documentation.
Running hugo does not remove generated files before building. This means that you should delete your public/ directory (or the publish directory you specified via flag or configuration file) before running the hugo command. If you do not remove these files, you run the risk of the wrong files (e.g., drafts or future posts) being left in the generated site.
A large part of being a developer is reading documentation. I obviously need to read more wisely.
After deleting public/ and regenerating my content, all is good once more. I will need to build a delete step into my build process to avoid this in the future.