Hi,
When the "resize" happens (or when the website opens in a small screen, ie: mobile phone), the original menu-items are re-populated to create the responsibe-menu-block.. After resizing back to original, the responsive-menu-block doesn't repopulated back to stock, because it is not necessary.
Resizing the browser is not a "REAL" case.. You notice this failure because you are testing your website to see how it looks like in mobile phones, right?. But how many "REAL" user will open the website in a 700px browser and then increase the browser size? No body..
Regards