Relationship Field ACF as Shortcode

acf-rs2.php

<?php //<~ don't add me in

add_shortcode( 'tl_related_lights', 'tl_related_lights_relationship' ); // Add your shortcode here
// Add Relationship  field between same CPT
function tl_related_lights_relationship() {

ob_start();
  
$posts = get_field('relationship_field_name');  // Add your ACF field in here

if( $posts ): ?>
    <ul>
    <?php foreach( $posts as $post): // variable must be called $post (IMPORTANT) ?>
        <?php setup_postdata($post); ?>
        <li>
            <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
        </li>
    <?php endforeach; ?>
    </ul>
    <?php wp_reset_postdata(); // IMPORTANT - reset the $post object so the rest of the page works correctly ?>
<?php endif; ?>

	<?php
	return ob_get_clean();
}

functions.php

<?php   //<~ don't add me in

add_shortcode( 'tl_related_lights', 'tl_related_lights_relationship' ); // Add your shortcode here
// Add Relationship ACF field between Projects and Lights
function tl_related_lights_relationship() {
	ob_start();
	
	$posts = get_field('tl_project_lights_link_used'); // Add your ACF field in here

	if( $posts ): ?>
		<ul>
		<?php foreach( $posts as $p ): // variable must NOT be called $post (IMPORTANT) ?>
		    <li>
		    	<a href="<?php echo get_permalink( $p->ID ); ?>"><?php echo get_the_title( $p->ID ); ?></a>
		    </li>
		<?php endforeach; ?>
		</ul>
	<?php endif; ?>
	
	
	<?php
	return ob_get_clean();
}

Also- not sure this is worth a fork/commit- but here is a sample of using your snippet to pull ACF wysiwyg content:

//shortcode for WYSIWYG field type modified by adamdexter
// see https://www.advancedcustomfields.com/resources/wysiwyg-editor/
add_shortcode( 'footnotes', 'footnotes_wysiwyg' ); // Add your shortcode here
// Registers function "footnotes_wysiwyg" as shortcode "footnotes"
function footnotes_wysiwyg() {
	ob_start();
	
	$posts = get_field('footnotes'); // Add your ACF field in here

	if( $posts ): ?>
		<div>
		    	<?php echo get_field('footnotes'); ?>
		</div>
	<?php endif; ?>
	
	
	<?php
	return ob_get_clean();
	
}