The caching mechanism is not truly effective, because in the second call $title argument is passed already processed, so it could be different than original one.
For example, with your profile member page, first time the function is called with $title = "Mike", second time with $title = "mike"...
It's not a real bug, but in Class XenForo_Link, method buildIntegerAndTitleUrlComponent(), there is an apparent unnecessary double call to method getTitleForUrl().
$title = self::getTitleForUrl($title, $romanize);
if ($title !== '')
{
# /item-title.id/ (where delimiter is '.')
return...