Search not finding some posts

I have a CPT called “letters” and it has a taxonomy called “letter tags”. I have added a taxonomy “letter ypres” to one of these posts but when I do a site wide search for “letter ypres” that post is not found. I know that none of my letter posts contain the words “letter” and “ypres.” Shouldn’t the search include posts with related tags? My search form requires both search terms to be present for a post or medium to show up in a search.