'date_query' argument takes an additional option that is ‘relation’ that can be AND or OR.
If we consider the relation OR your code, that use exactly same date for ‘before’ and ‘after’ with ‘inclusive’ set to true, should return all your posts: posts before a date + posts after that date + posts in that date.
If we consider the relation AND, it can be summarized in post_date >= date && post_date <= date and that it’s true for date itself, but probably there’s something on date query class (I’ve not digged in the core) that prevents such a query, that to be honest, make no sense to me.
The reason is simple: if you want to query a specific date you don’t need to use after/before at all, just create an array with the year, month and day you want to display
$args = array(
'date_query' => array(
array(
'year' => 2012,
'month' => 12,
'day' => 12,
),
),
);
$query = new WP_Query( $args );
The code above is taken from second example for date query parameters in Codex.