<?php
/** Create Title and Slug */
function acf_title( $value, $post_id, $field ) {
	if ( get_post_type( $post_id ) === 'staff' ) {

		$new_title = get_field( 'first_name', $post_id ) . ' ' . $value;
		$new_slug  = sanitize_title( $new_title );

		wp_update_post(
			array(
				'ID'         => $post_id,
				'post_title' => $new_title,
				'post_name'  => $new_slug,
			)
		);
	}
	return $value;
}
add_filter( 'acf/update_value/name=last_name', 'acf_title', 10, 3 );