It is also present in German Wikipedia and not present in French. Please enwiki & frwiki teach us how you learned to avoid it.

Tracing where the contents of contentSub come from:

				<div id="contentSub"<?php $this->html( 'userlangattributes' ) ?>><?php
					$this->html( 'subtitle' )
				?></div>

'subtitle' is set here:

		$tpl->set( 'subtitle', $subpagestr . $out->getSubtitle() );

$subpagestr is empty unless a page has subpages, so only this part matters:

	public function getSubtitle() {
		return implode( "<br />\n\t\t\t\t", $this->mSubtitle );
	}

mSubtitle is added to by:

	public function addSubtitle( $str ) {
		if ( $str instanceof Message ) {
			$this->mSubtitle[] = $str->setContext( $this->getContext() )->parse();
		} else {
			$this->mSubtitle[] = $str;
		}
	}

So, I suspect that something is calling addSubtitle() with either an empty string, or a message that ruwiki changes to be empty. (Note that if a message is shown in wiki content language, rather than user language, uselang=qqx will not reveal its message key.)

We could probably easily fix this by filtering out empty elements before calling implode(). But it would be good to find out first why an empty element is added, and whether that's intentional.

I see the same problem at:

• https://pl.wikipedia.org/w/index.php?title=Foo&action=info
• https://hu.wikipedia.org/w/index.php?title=Foo&action=info

But not at:

• https://fr.wikipedia.org/w/index.php?title=Foo&action=info

A common factor seems to be having FlaggedRevs enabled…

I can reproduce on enwiki too, but only for pages which have FlaggedRevs-based "pending changes" enabled, for example https://en.wikipedia.org/w/index.php?title=Linux&action=info.

Change 339821 had a related patch set uploaded (by Bartosz Dziewoński):
Don't add empty review notice to page subtitle (contentSub)

https://gerrit.wikimedia.org/r/339821

Change 339821 merged by jenkins-bot:
Don't add empty review notice to page subtitle (contentSub)

https://gerrit.wikimedia.org/r/339821

This issue has been fixed and the fix will be deployed to Wikimedia wikis this week, 28 February – 2 March, per https://www.mediawiki.org/wiki/MediaWiki_1.29/Roadmap.