Relative links begin like [[/ or [[../ (and transclusions {{/ or {{../).
https://meta.wikimedia.org/wiki/Help:Links#Relative_links
Currently we break them: https://www.mediawiki.org/w/index.php?title=Extension%3AConfirmAccount&diff=1107033&oldid=1012488 (see [[/Integration etc.).
Instead they need to be expanded (hopefully there's an API for that). It's fine to just skip them for now.