So when you say you selected a subcategory and remained on the same listing, that implies you did a search prior, right? And does that mean a search with a category selected that is a supercategory/ancestor of that subcategory? I'm just asking because if a supercategory is selected, of course it will display all posts in that category and all its descendant categories, and then when a subcategory is clicked, if all the categories listed also belong to that subcategory, then nothing will change. So I guess I should ask: If you select its supercategory only and search, do you get extra results, and if not, are there posts belonging only to the supercategory or to the subcategory's siblings and not to the subcategory?
Another thing to check: Does the page actually reload when you click "search"? I just want to make sure it's really being submitted.