Add tags to a "pending" post before publishing

Question

I use a third-party plugin that creates custom posts (adverts) from the front-end in two steps: 1) after the content adding the author must push a "Preview" button and 2) only after that he/she already can save the post. After the first step the post is saved with a pending (advert_tmp) status, and after the second he become "publish"-ed.

This is what I want: between these two steps to convert/save a meta_key values as the post tags. After some research I came to the next partially working function:

add_action('save_post', 'set_advert_tags');
function set_advert_tags( $post_id ) {
   if ( 'advert' == get_post_type() ) {
      $tags = get_post_meta( $post_id, 'advert_tags_field', true );
      $tags = explode( ',', $tags );
      if(!empty( $tags ))
         wp_set_object_terms( $post_id, $tags, 'advert_tag' );
   }
}

What is wrong: the function doesn't create tags nor after the first step, nor after the second step, but it works when I update the post from the back-end. And here I have another problem: I can't add or assign new tags to the post from the back-end because my function disturb somehow this action, only the meta_key values are assigned as post tags.

Answer 1

The @PieterGoosen's answer saved my life:

add_action( 'transition_post_status', function ( $new_status, $old_status, $post ) {
   if( 'publish' == $new_status && 'advert_tmp' == $old_status && $post->post_type == 'advert' ) {
      $tags = get_post_meta( $post->ID, 'advert_tags_field', true );
      $tags = explode( ',', $tags );
      if( !empty( $tags ) ) {
         wp_set_object_terms( $post->ID, $tags, 'advert_tag' );
         delete_post_meta( $post->ID, 'advert_tags_field' );
      }
    }
}, 10, 3 );