Assuming this is about WS Export.

Yes, you are correct. Thanks for adding the tag, @Aklapper.

The reason that this behaviour has changed since the switch to Parsoid HTML is that links within the same document are now prefixed with the document name, and they used to not be. This means that it's now more complicated to determine which links are same-document ones. They used be be e.g. href="#foo" but Parsoid adds the page name e.g. href="Lorem#foo". Is this intentional by Parsoid? It means that the document can't be used with any other name (e.g. downloading it to Lorem.html breaks the links).

If the page name is added to the link, I think the first elseif would catch it and build a new link as expected. Could you investigate if the problem is not with the node. I don’t see any title attribute in the neighborhood of href="#foo" and one is needed :

$title = Util::encodeString( $node->getAttribute( 'title' ) ) . '.xhtml';

Assuming the link is prefixed with the document name, why can’t we link to that document whether it is the same or not ?

Something like this may work :

	 * change the internal links
	 */
	protected function setLinks( DOMDocument $dom ) {
		$list = $dom->getElementsByTagName( 'a' );
		/** @var DOMElement $node */
		foreach ( $list as $node ) {
			$href = $node->getAttribute( 'href' );
			$title = Util::encodeString( $node->getAttribute( 'title' ) ) . '.xhtml';
                        $pos = strpos( $href, '#' );
                        if (strlen($title) < 7) && ( $pos !== false ) {
                                $title = Util::encodeString( substr( $href, 0, $pos - 1 )) . '.xhtml';
                        }
			if ( substr( $href, 0, 1 ) === '#' ) {
				continue;   //there was no array_search to get the transformed id ??
			} elseif ( in_array( $title, $this->linksList ) ) {
				//$pos = strpos( $href, '#' );
				if ( $pos !== false ) {
					$anchor = substr( $href, $pos + 1 );
					$title .= '#' . array_search( $anchor, PageParser::getIds() );
				}

Is this intentional by Parsoid?

I would say yeah. There're some hints at it in T69486 but links in Parsoid are rendered relative to the <base href="//en.wikipedia.org/wiki/"/> element in the head of the document.

CommTech is not planning on working on this.