tax_query not returning posts that multiple terms are applied to

Hi, maybe I am misusing the tax_query parameter set, but here is my code:

$posts = get_posts( array(
    'post_type' => get_post_type($current_id),
	'tax_query' => array(
		array(
			'taxonomy' => $tax,
			'terms' => $term_id,
			'field' => 'id',
			'operator' => 'IN',
			'include_children' => true
		)
	)
) );

What this is supposed to do is return all posts that belong to term $term_id of the custom taxonomy $tax. Seems simple enough and it does return such posts, except those that also belong to another term within the same taxonomy. It’s maddening and either this is a bug or I am misunderstanding how the API works.

Can someone please help me keep from pulling the rest of my hair out?