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Line 459 onwards of SpecialMergeAccount.php is at fault


See 466 where it's always a local title object:

		$userPageName = Title::makeTitleSafe( NS_USER, $this->mUserName );
		return Xml::element( 'a',
			array(
				'href' => $userPageName->getFullUrl(),
				'title' => $this->msg( 'centralauth-foreign-link',
					$this->mUserName,
					$hostname )->text(),
			),
			$hostname );

Vs SpecialCentralAuth.php

		$wikiname = $wiki->getDisplayName();
		return $this->foreignLink(
			$wiki,
			'User:' . $this->mUserName,
			$wikiname,
			$this->msg( 'centralauth-foreign-link', $this->mUserName, $wikiname )->text() );

Related URL: https://gerrit.wikimedia.org/r/62839 (Gerrit Change If2c59da4342e0a149b775b4588353eff0106a708)

Caused by Gerrit change #52337

Fixed in master by Reedy's code.