"dawhoo" was on to something, but it had nothing to do with the DB info. That config file (app/etc/env.php) contains config settings of a sensitive nature and is the place where sensitive config values are dumped if you do an export. After I inadvertently dumped all settings and locked them all, I recovered by removing the exported values from the app/etc/config.php file. However, the sensitive ones were exported to app/etc/env.php, which I was not aware of. We use git for deployments and it's advised NOT to have app/etc/env.php in git, so I didn't even notice the change to the file.