How to display field data?

Doesn’t matter! After some digging, I found how to do it. Thanks!

<?php
            $menu = wp_get_nav_menu_object ( 'header-navigation' );
            $menu_items = wp_get_nav_menu_items( $menu->term_id );
        ?>
        <?php foreach( $menu_items as $menu_item ) { ?>
            Name
            <?php get_field( 'additional_field', $menu_item->ID ); ?>
        <?php } ?>

You could also output the data by using a custom Walker.

Heres a tutorial on how to add the menu descriptions to your walker: https://www.wpbeginner.com/wp-themes/how-to-add-menu-descriptions-in-your-wordpress-themes/

Using the same logic you could output the ACF field data using the_field('your_field', $item)