<?php

/* check link for reference:
/* https://www.advancedcustomfields.com/resources/get_sub_field/ */

$array = array();

if( have_rows('parent_field') ):

    while( have_rows('parent_field') ) : the_row();

        $value = get_sub_field('sub_field');
                $array = array_merge($array, $value);

    endwhile;

endif;

var_dump($array)

?>