Global nav not generating "static selected" class on selected tab

Question

Sharepoint 2010 - the global nav on my site for the selected tab is displaying like this:

<ul class="static">
<li class="static">
<li class="static">
<a class="static menu-item" href="/sites/misc/CMI2/Lists/Posts/link.aspx">

rather than switching to selected (like this)

<ul class="static">
<li class="static">
<li class="static selected">
<a class="static selected menu-item" href="/sites/misc/CMI2/distribn/SitePages/Home.aspx">

and I have no idea why. It had been working previously...