I tried your code with a little bit changes and it’s working,
I think you have missed 'field_groups' => array(YOUR_FIELD_GROUP_ID), in your acf_form() , Try adding the id of your field group to the acf_form()
<?php
function woocommerce_variation_options_acf_form($loop, $variation_data, $variation) {
add_action('admin_head','acf_form_head',20);
$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',
'field_groups' => array(28),
'return' => add_query_arg( array( 'post_id' => $post_id, 'updated' => 'true'), get_permalink() ),
));
// var_dump( $variation_id );
?>
<script type="text/javascript">
(function($) {
// setup fields
acf.do_action('append', $('#popup-id'));
})(jQuery);
</script>
</td></tr>
<?php
}
add_action('admin_head','acf_form_head',20);
add_action('woocommerce_product_after_variable_attributes','woocommerce_variation_options_acf_form',99);
/**
In WooCommerce : includes/admin/meta-boxes/views/html-variation-admin.php
*/
do_action( 'woocommerce_product_after_variable_attributes', $loop, $variation_data, $variation ); ?>
try this and use your own field group id instead of ’28’ in 'field_groups' => array(28)
