i'm sorry but
i have just tried now, from a brand new site, with nothing installed but joomla, joomsef, and the theme i use (japurity 2 for menus), and it DOES bug.
even from a brand new "from scratch" joomla site (1.5.18).
procedure i used, so you can try yourself:
i installed joomla 1.5.18 fr
installed ja purity2
installed joomsef
activated SEF option in joomla config
took one of the menus (joomla overview), with child menu, turned the parent into separator
tried it, and it does, indeed, link to homepage....
so, as the only think that differs from a from scratch joomla, is the template, do you think it might come from there? even though it uses the mod_mainmenu: menu module = mainmenu, menu type= mega menu.