By 'inspect the CSS for a page' you mean you checked by developer toolbar if a span is wrapped around the link?

I had another look on the wikis i have here, and they have all this curid span, so far see. 
(example: when you go to https://www.dokuwiki.org/extensions , you will see in the sidebar a typefaced bold entry 'extensions'. That entry i do also expect in the indexmenu list. When you look by developer toolbar you will see how the code looks.)

Can you provide the indexmenu syntax you used? And when you can provide a copy of a part of the tree that contains this current page, i would welcome it, it will give me a bit more detailed view of the case.