The method prettifyUrls in api/FormatHtml.php tries to match the following RegEx:
'/(https?:)?\/\/((\w+)\.wikipedia\.org\/w\/index\.php\?title=(.*)&oldid=(.*))/
But since T131344 we now support other sites than Wikipedia. For ru: we are getting from Wikivoyage, resulting in links like //ru.wikipedia.org/w/index.php?title=Title&oldid=162698 which do not match