Thanks, that was very helpful. Just to be precise and for those who will find this post useful. The_field prints out clean html so the right way will be:
foreach($product_terms as $term){ $category_id_tag = 'post_tag_'.$term->term_id;
echo '<a href="'.get_term_link($term->slug, 'post_tag').'" title="';
the_field('title_page', $category_id_tag); echo '"><span>'.$term->name.'</span>
<span>'.$tag_title.'</span>
</a> ';
}