Wordpress: ACF-Field as CPT Title

functions.php

// inspired by: https://gist.github.com/rveitch/9018669face1686e74aaa68026856f36
// add title to CPTs which don't provide a title (useful for the relationship field (https://www.advancedcustomfields.com/resources/relationship/))

function sync_acf_post_title($post_id, $post, $update) {

	$post_type = get_post_type($post_id);

	// check for the current CPT
	if($post_type === "cpt_name_1") {

		// get the field you want to save in the title
		$title = get_field('acf_field_name', $post_id);

	} else if($post_type === "cpt_name_2") {

		$title = get_field('acf_field_name', $post_id);

	} else {

    		//if it's not a CPT, the title should be saved as usual
		$title = $post->post_title;

 	}

	$content = array(
	  'ID' => $post_id,
	  'post_title' => $title
	);

	// to prevent a loop
	remove_action('save_post', 'sync_acf_post_title');
	wp_update_post($content);

}

// is triggered when a user presses the update or publish button
add_action('save_post', 'sync_acf_post_title', 10, 3);

Excellent, thank you very much !