sleep injection fails on `NOT EXISTS` queries

[valhallasw]

Example: https://quarry.wmflabs.org/query/52344

SELECT page_namespace, page_title, il_to
	FROM imagelinks
	JOIN page on page_id = il_from
	WHERE
		NOT EXISTS (SELECT * FROM image WHERE img_name = il_to)
   	AND NOT EXISTS (SELECT * FROM commonswiki_p.image WHERE img_name = il_to)
		AND NOT EXISTS (SELECT * FROM page WHERE page_namespace = 6
					AND page_title = il_to)
		AND NOT EXISTS (SELECT * FROM commonswiki_p.page WHERE page_namespace = 6
					AND page_title = il_to)
	ORDER BY page_namespace ASC LIMIT 1000

or, a more minimal example,

SELECT page_namespace, page_title, il_to
	FROM imagelinks
	JOIN page on page_id = il_from
	WHERE
		NOT EXISTS (SELECT * FROM image WHERE img_name = il_to)
	ORDER BY page_namespace ASC LIMIT 1000

The sleep seems to be injected into the subquery rather than the main query, and this seems to break the syntax.