Execute function after a post has been published

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

        $post_type = get_post_type($post_id);
        if ( "post" != $post_type ) return;


        $video_code_val = get_post_meta( $post_id, 'video_code', true );

        // Check if the video_code field has a value.
        if ( ! empty( $video_code_val ) ){

         //if it does, set post format
         set_post_format( $post_id, 'video' );

        }

}

add_action( 'save_post', 'check_meta', 999, 3 )

Maybe also try other actions…

If the post is saved via published:

add_action( 'publish_post', 'check_meta', 999, 3 );

or if on if a post is saved as draft:

add_action( 'draft_to_publish', 'check_meta', 999, 3 );

or this updated_post_meta action might work:

add_action( 'updated_post_meta', 'check_meta', 999, 3 );