Checkbox List and Select Advanced (multiple true) returning a string

I have both checkbox_list and select_advanced (which has multiple => true) returning a string instead of array even though multiple choices are selected. Why?

// ----- field
[
	'name'				=> esc_html__( 'My Checkbox', 'textdomain' ),
	'label_description'	=> esc_html__( 'My Checkbox', 'textdomain' ),
	'id'				=> 'checkbox_list_field_id',
	'type'				=> 'checkbox_list',
	'inline'			=> false,
	'select_all_none'	=> false,
	'options'			=> [
		'option_1'	=> esc_html__( 'Option 1', 'textdomain' ),
		'banana'	=> esc_html__( 'Banana', 'textdomain' ),
		'potato'	=> esc_html__( 'Potato', 'textdomain' ),
	],
],
// ----- then in code
$args = [
	'post_type' => 'my_custom_post',
	'author__in' => $array_of_user_ids,
	'posts_per_page' => -1
];

$query = new WP_Query( $args );

if ( $query->have_posts() ) {
	while ( $query->have_posts() ) {
		$query->the_post();
		$checkbox_list = rwmb_meta( 'checkbox_list_field_id' );
		var_dump($checkbox_list);
		if ( !is_array( $checkbox_list ) ) {
			echo '<h3 style="z-index:100000000000; background: red; padding: 60px; display: block;">NOT ARRAY</h3>';
			die();
		} else {
			echo '<h3 style="z-index:100000000000; background: red; padding: 60px; display: block;">YES ARRAY</h3>';
			die();
		}	
	}

	wp_reset_postdata();
}