When installed as a Must-Use plugin, URL paths default to the /plugins directory instead of the /mu-plugins directory causing all enqueued stylesheets and scripts to 404. Applying a second argument to each call to plugins_url containing the plugin_dir_path (which contains the correct /mu-plugins path) corrects these URLs.
When installed as a Must-Use plugin, URL paths default to the /plugins directory instead of the /mu-plugins directory causing all enqueued stylesheets and scripts to 404. Applying a second argument to each call to plugins_url containing the plugin_dir_path (which contains the correct /mu-plugins path) corrects these URLs.