Save title as custom field on custom post type

This will automatically update a meta named post_title to the value of the post’s title, no matter where you save the post from:

function wpse_275785_save_title_as_meta( $post_id, $post, $update ) {
    update_post_meta( $post_id, 'post_title', $post->post_title );
}
add_action( 'save_post', 'wpse_275785_save_title_as_meta', 99, 3 );