I created a view: [wpv-view name="custom-post-grid" catslug="free-sheet-music"] and the results are pulling in posts from categories outside of the slug. These posts are not categorized in the slug at all. I added a screenshot of my view.
Looking at the debug info I do not see the category being filtered.
SELECT SQL_CALC_FOUND_ROWS wp_posts.ID FROM wp_posts WHERE 1=1 AND wp_posts.ID NOT IN (34933,35273,34713,34473,32190,31965,31831,31775,31710,31571,31333,31332,6333) AND wp_posts.post_type = 'post' AND ((wp_posts.post_status = 'publish')) ORDER BY wp_posts.menu_order, wp_posts.post_date DESC LIMIT 0, 5