WP_Query not returning custom post type

WP_Query does not accept a comma delimited string for the post_type argument, it must be one of either a string (for one post_type) or an array (for one-or-more post_type).

$args = array(
    'post_type' => 'post'
);

$query = new WP_Query($args);

OR

$args = array(
    'post_type' => 
    array(
        'post'
    )
);

$query = new WP_Query($args);

OR

$args = array(
    'post_type' => 
    array(
        'post',
        'page',
        'movie',
        'book'
    )
);

$query = new WP_Query($args);

Useful reading:

• https://developer.wordpress.org/reference/classes/wp_query/#post-type-parameters