It works on most other places because in the templates it appears to have
Code:
<xen:username user="$user" itemprop="name" rich="true" />
or something similiar in it but for the sidebar_visitor_panel it doesn't. Will probably require a template edit. I'll play with it and see if I can figure out what (but mine will be trial and error - hopefully Brogan, Jake or someone more familiar with it will chime in).