tax_query OR with empty result returns all posts

Question

I'm running a WP_Query with a tax_query with two tax_query statements with an OR relationship.

$tax_query_terms = array(
                           array('taxonomy' => 'post_tag',
                                 'terms' => array(695),
                                  'field' => 'id'),
                           'relation' => 'OR',
                           array('taxonomy' => 'advisers',
                                 'terms' => array(301),
                                 'field' => 'id')
                        );

$related_articles = new WP_Query(array('tax_query' => $tax_query_terms,
                                             'post__not_in' => array(get_the_ID()), 
                                             'posts_per_page' => 3,
                                             'orderby' => 'date',
                                             'order' => 'DESC'));

When there are one or more posts for either term, it works correctly. When there are no posts matching either term, it returns all posts.

I would expect that if there are no posts matching either tax_query statement, then the query should return no posts. Am I wrong in this assumption?

(This seems to have been fixed for WordPress 3.5, but potentially has been re-introduced with the nested tax_queries for WordPress 4.1. - see https://core.trac.wordpress.org/ticket/20604 )

Am I misunderstanding this here, or should I file a bug on trac?