At the present, a diff uses "<-" and "->" to direct users to previous and subsequent edits, rather than "←" and "→". If you want a quick view, see https://en.wikinews.org/w/index.php?title=Wikinews:Water_cooler/proposals&diff=prev&oldid=4377477 and compare it with https://en.wikiversity.org/w/index.php?title=Wikiversity:Main_Page&diff=prev&oldid=1137405 or https://simple.wiktionary.org/w/index.php?title=chestnut&type=revision&diff=400758&oldid=400756 or https://es.wikinews.org/w/index.php?title=Usuario%3AKoavf&type=revision&diff=586304&oldid=296778 or https://outreach.wikimedia.org/w/index.php?title=Main_Page&type=revision&diff=150960&oldid=120911 or https://wikisource.org/w/index.php?title=User:Angela&diff=prev&oldid=542738 or really any other project as far as I'm aware. Somehow, en.wn seems to be the only project where I'm seeing this. It's a pretty low-importance fix but it's also probably very simple to deploy and it irks me every time I see a diff on en.wn.
Thanks for reporting this!
Going to https://en.wikinews.org/w/index.php?title=Wikinews:Water_cooler/proposals&diff=prev&oldid=4377477&uselang=qqx you see that the string IDs are previousdiff and nextdiff.
Looking at https://en.wikinews.org/wiki/MediaWiki:Previousdiff and https://en.wikinews.org/wiki/MediaWiki:Nextdiff you can see that these are customized messages on en.wn. Hence this needs to be changed and fixed on-wiki, and changing on-wiki content is out of scope for Phabricator.