function woocommerce_variation_options_acf_form($loop, $variation_data, $variation) {

  $variation_id = $variation->ID;
  ?>
  <tr><td colspan="2">
  <style>
  table.data_table td #poststuff { padding: 0; min-width: 1em;}
  table.data_table td .postbox { min-width: 1em;}
  table.data_table td div.acf_postbox div.field_type-true_false p.label { display: inline; }
  table.data_table td div.acf_postbox div.field_type-true_false ul {display: inline-block;}
  #variable_product_options .woocommerce_variation table td input[type='checkbox'] { min-width: 1em!important;}
  </style>
  <?php
  acf_form(array(
      'post_id' => $variation_id,
      'form' => false,
      'label_placement' => 'top',
      'instruction_placement' => 'label',
      'return' => add_query_arg( array( 'post_id' => $post_id, 'updated' => 'true'), get_permalink() ),
      ));

      ?>
      <script type="text/javascript">
(function($) {
  // setup fields
  acf.do_action('append', $('#popup-id'));
})(jQuery); 
</script>
</td></tr>
<?php

}
add_action('woocommerce_product_after_variable_attributes','woocommerce_variation_options_acf_form',19,3);

add_action('admin_head','acf_form_head',20);

/**
In WooCommerce : includes/admin/meta-boxes/views/html-variation-admin.php
<?php do_action( 'woocommerce_product_after_variable_attributes', $loop, $variation_data, $variation ); ?>
**/