In HTML validation, <menuitem> without a closing tag results in an error "Unclosed element menuitem", while <menuitem /> syntax (in proper contexts) is flagged as invalid ("Self-closing syntax (/>) used on a non-void HTML element.") while the element is listed among void elements (http://www.w3.org/html/wg/drafts/html/master/syntax.html#void-elements).
As of now, menuitem is not supported by browsers, and using it in an HTML documents results in the nesting of subsequent <menuitem>s.
This can be a bug to fix or (better) an occasion for filing a bug to the HTML5's spec, to list <menuitem> as non-void in order to maintain backwards compatibility.