ACF true/false loop

Hello, I have a problem.
I’ve created a custom post type Portfolio in my site and with ACF I’ve created 4 extra fields :
2 for images (“imagen” e “imagen_destacados”), a text field (“description”) and a true/ false field (“selected”).
I’ve also added a taxonomy to the custom post type named “category” with five options.

function create_posttype() {

	register_post_type( 'portfolio',
	// CPT Options
		array(
			'labels' => array(
				'name' => __( 'portfolio' ),
				'singular_name' => __( 'portfolio' )
			),
			'public' => true,
			'has_archive' => true,
			'rewrite' => array('slug' => 'portfolio'),
		)
	);
}

add_action( 'init', 'create_posttype' );

function custom_post_type() {

// Set UI labels for Custom Post Type
	$labels = array(
		'name'                => _x( 'portfolio', 'Post Type General Name', 'calduchstudio' ),
		'singular_name'       => _x( 'portfolio', 'Post Type Singular Name', 'calduchstudio' ),
		'menu_name'           => __( 'portfolio', 'calduchstudio' ),
		'parent_item_colon'   => __( 'Parent portfolio', 'calduchstudio' ),
		'all_items'           => __( 'All portfolio', 'calduchstudio' ),
		'view_item'           => __( 'View portfolio', 'calduchstudio' ),
		'add_new_item'        => __( 'Add New portfolio', 'calduchstudio' ),
		'add_new'             => __( 'Add New', 'calduchstudio' ),
		'edit_item'           => __( 'Edit portfolio', 'calduchstudio' ),
		'update_item'         => __( 'Update portfolio', 'calduchstudio' ),
		'search_items'        => __( 'Search portfolio', 'calduchstudio' ),
		'not_found'           => __( 'Not Found', 'calduchstudio' ),
		'not_found_in_trash'  => __( 'Not found in Trash', 'calduchstudio' ),
	);
	
// Set other options for Custom Post Type
	
	$args = array(
		'label'               => __( 'portfolio', 'calduchstudio' ),
		'description'         => __( 'portfolio news and reviews', 'calduchstudio' ),
		'labels'              => $labels,
		// Features this CPT supports in Post Editor
		'supports'            => array( 'title', 'editor', 'author', 'thumbnail', 'revisions', ),
		// You can associate this CPT with a taxonomy or custom taxonomy. 
		'taxonomies'          => array( 'category' ),
		/* A hierarchical CPT is like Pages and can have
		* Parent and child items. A non-hierarchical CPT
		* is like Posts.
		*/	
		'hierarchical'        => false,
		'public'              => true,
		'show_ui'             => true,
		'show_in_menu'        => true,
		'show_in_nav_menus'   => true,
		'show_in_admin_bar'   => true,
		'menu_position'       => 5,
		'can_export'          => true,
		'has_archive'         => true,
		'exclude_from_search' => false,
		'publicly_queryable'  => true,
		'capability_type'     => 'page',
	);
	
	// Registering your Custom Post Type
	register_post_type( 'portfolio', $args );

}

/* Hook into the 'init' action so that the function
* Containing our post type registration is not 
* unnecessarily executed. 
*/

//* Link e4gf_events CPT to categories taxonomy
add_action( 'init', 'sk_add_category_taxonomy_to_portfolio' );
function sk_add_category_taxonomy_to_portfolio() {
	register_taxonomy_for_object_type( 'category', 'portfolio' );
}

I’ve created this “archive-portfolio.php”:

<?php 

/**
 *
 * @author  Ivan Ortiz & Xavier Guerrero Calduch
 */

//* Template Name: Portfolio
//* The template for displaying the portfolio of our website.

get_header(); ?>

<div id="primary" class="content-area">
	<main>

		<?php

			$args = array( 'post_type' =>portfolio,
						   'post_status'    => 'publish'
						  );

			$query = new WP_Query( $args ); 

			$i=0;

			if ( $query->have_posts()) : while ( $query->have_posts() ) : $query->the_post();

				$selected = get_field('selected');

				if($selected):

					$image = get_field('imagen_destacados');

					if( !empty($image) ):
					?>

						<article class="article-portfolio">
							<a href="<?php the_permalink(); ?>">
								<img src="<?php echo $image['url']; ?>" alt="<?php echo $image['alt']; ?>" />
							</a>
						</article>
					<?php endif; ?>

				<?php endif; ?>

			 	<?php endwhile; ?>

			<?php endif; ?>

	</main>
</div>

<?php get_footer(); ?>

The matter is that when I try to check if the selecetd field is checked or not in some categories it Works and in others not.
Could somebody tell me What happens?
I Would really appreciate it.
Thanks in advance!