I’ve just tested it on my installation but failed to reproduce the issue. Maybe there’s something on your site that prevents ACF from formatting the date. Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Sixteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
If the issue persists, could you please share the JSON or XML export file of your field group so I can test it out on my installation?