The menu item is using "CSS Regions". By default that is not enabled in Reflow because Regions are brand new and not widely supported in all browsers yet. You can learn more about css regions and their browser support here: http://html.adobe.com/webplatform/layout/regions/
If you want to play with CSS Regions in Reflow or you think they may work for your project and target users you can turn them on by going to "View > Shinny Web Features..." and click the toggle switch to turn on support in Reflow:
You can learn more about what the CSS Regions switch turns on in Reflow here: http://html.adobe.com/edge/reflow/shiny-web-features.html
Hope that helps and thanks for using Reflow!
Chris