I have investigated a bit more and it does seem to be doing partial matching for custom fields. But the problem is actually much more mysterious – it is only certain words that aren’t doing partial matches, and it doesn’t matter if the content is stored in wp_posts.post_content or wp_postmeta.meta_value. Nor does it matter (nor should it matter) which attachment it is associated with. I really can’t find a pattern.
Are there certain characters or suffixes that are excluded or treated differently?