Auto-populate post title field with post ID in WordPress. Source: http://support.advancedcustomfields.com/forums/topic/createupdate-post-title-from-acf-fields/

auto-populate-post-title-with-post-id.php

<?php

//* Auto-populate post title field with post ID
add_action('save_post', 'customprefix_update_post_title' );
function customprefix_update_post_title($post_id) {

  $my_post = array();
  $my_post['ID'] = $post_id;
  
  // Auto-populate post title field only if post type = post
  if ( get_post_type() == 'post' ) {
    $my_post['post_title'] = get_the_ID();
  }
    
  // Avoid infinite loops
  // https://codex.wordpress.org/Plugin_API/Action_Reference/save_post#Avoiding_infinite_loops
  remove_action( 'save_post', 'customprefix_update_post_title' );
  wp_update_post($my_post);
  add_action( 'save_post', 'customprefix_update_post_title' );

}