Can not save multiple product variation fields - wordpress

Question

When I save the first product variation fields they are saved, but when I try to save the second, third, forth and further they do not get saved.
I am filling out the SKU for every product variation as well, so it should work. But obv it is something I am missing here? This is the latest WordPress and latest WooCommerce version.
//Display Fields
add_action( 'woocommerce_product_after_variable_attributes', 'variable_fields', 10, 3 );
//Save variation fields
add_action( 'woocommerce_save_product_variation', 'save_variable_fields', 10, 1 );
/**
* Create new fields for variations
*
*/
function variable_fields( $loop, $variation_data, $variation ) {
?>
<tr>
<td>
<?php
// Textarea
woocommerce_wp_textarea_input(
array(
'id' => '_textarea['.$loop.']',
'label' => __( 'Contains', 'woocommerce' ),
'placeholder' => '',
'description' => __( '<br />Contains', 'woocommerce' ),
'value' => get_post_meta( $variation->ID, '_textarea', true ),
)
);
?>
</td>
</tr>
<tr>
<td>
<?php
// Textarea
woocommerce_wp_textarea_input(
array(
'id' => '_textarea_2['.$loop.']',
'label' => __( 'Observation message', 'woocommerce' ),
'placeholder' => '',
'description' => __( '<br />Observation message', 'woocommerce' ),
'value' => get_post_meta( $variation->ID, '_textarea_2', true ),
)
);
?>
</td>
</tr>
<?php
}
/**
* Save new fields for variations
*
*/
function save_variable_fields( $post_id ) {
if (isset( $_POST['variable_sku'] ) ) :
$variable_sku = $_POST['variable_sku'];
$variable_post_id = $_POST['variable_post_id'];
// Textarea
$_textarea = $_POST['_textarea'];
for ( $i = 0; $i < sizeof( $variable_sku ); $i++ ) :
$variation_id = (int) $variable_post_id[$i];
if ( isset( $_textarea[$i] ) ) {
update_post_meta( $variation_id, '_textarea', stripslashes( $_textarea[$i] ) );
}
endfor;
// Textarea
$_textarea_2 = $_POST['_textarea_2'];
for ( $i = 0; $i < sizeof( $variable_sku ); $i++ ) :
$variation_id = (int) $variable_post_id[$i];
if ( isset( $_textarea_2[$i] ) ) {
update_post_meta( $variation_id, '_textarea_2', stripslashes( $_textarea_2[$i] ) );
}
endfor;
endif;
}

Answer 1

I've modified action & function for saving:
//Save variation fields
add_action( 'woocommerce_save_product_variation', 'save_variable_fields', 10, 2 );
/**
* Save new fields for variations
*
*/
function save_variable_fields( $variation_id, $i ) {
if ( empty( $variation_id ) ) return;
if ( isset( $_POST['_textarea'][$i] ) ) {
update_post_meta( $variation_id, '_textarea', stripslashes( $_POST['_textarea'][$i] ) );
}
if ( isset( $_POST['_textarea_2'][$i] ) ) {
update_post_meta( $variation_id, '_textarea_2', stripslashes( $_POST['_textarea_2'][$i] ) );
}
}