In the ProofreadPage page namespace, the "header" and "body" are joined with a double newline \n\n before being fed to the parser:

		$wikitextContent = new WikitextContent(
			$this->header->getText() . "\n\n" . $this->body->getText() .
				$this->footer->getText()
		);
		$parserOutput = $wikitextContent->getParserOutput( $title, $revId, $options, $generateHtml );

https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/ProofreadPage/+/refs/heads/master/includes/Page/PageContent.php#280

This is presumably intended to allow this:

to enter the parser like this:

Header text

Body text

and come out as two paragraphs.

However, when the header contains a list item, this is wrong:

which enters the parser like this:

* Item Foo

** Sub item bar

rather than:

* Item Foo
** Sub item bar

This causes mis-rendering for pages like this, which then has no way to keep the header and body part of the same list.

I am unsure if a straight change from \n\n to \n would cause major breakage, as it's very rare, at least at enWS for a header to end with inline content. They normally contain block content, but this is not guaranteed.

An alternative (if ugly) solution is a magic word or similar to suppress one of the \ns in the cases where it causes an issue.