custom field value isn't stored in variation_data object - wordpress

Question

i have following issue with woocommerce/dokan:
I created a custom field for product variation:
custom field in wordpress backend
It works fine, the values are storing and i can change the value at the backend. Therefor i added following lines of code into function.php of theme file:
function variation_settings_fields( $loop, $variation_data, $variation ) {
// Text Field
woocommerce_wp_text_input(
array(
'id' => '_text_field[' . $variation->ID . ']',
'label' => __( 'My Text Field', 'woocommerce' ),
'placeholder' => 'http://',
'desc_tip' => 'true',
'description' => __( 'Enter the custom value here.', 'woocommerce' ),
'value' => get_post_meta( $variation->ID, '_text_field', true )
)
);
}
/**
* Save new fields for variations
*
*/
function save_variation_settings_fields( $post_id ) {
// Text Field
$text_field = $_POST['_text_field'][ $post_id ];
if( ! empty( $text_field ) ) {
update_post_meta( $post_id, '_text_field', esc_attr( $text_field ) );
}
}
// Add New Variation Settings
add_filter( 'woocommerce_available_variation', 'load_variation_settings_fields' );
/**
* Add custom fields for variations
*
*/
function load_variation_settings_fields( $variations ) {
// duplicate the line for each field
$variations['text_field'] = get_post_meta( $variations[ 'variation_id' ], '_text_field', true );
return $variations;
}
So far, so good. I can set a value and the value can be saved over the backend.
Now i want to change the custom field for this product variation in dokan vendor dashboard. I duplicated html-product-variation.php file into child theme and added the new field:
dokan vendor dashboard product variation
with following code:
<div>
<p class="dokan-form-group">
<label><?php esc_html_e( 'Custom Textfield', 'dokan' ); ?></label>
<textarea class="dokan-form-control" name="_text_field[<?php echo esc_attr( $loop ); ?>]" rows="3" style="width:100%;"><?php echo isset( $variation_data['_text_field'] ) ? esc_textarea( $variation_data['_text_field'] ) : ''; ?></textarea>
</p>
</div>
The problem is, that the value, which i set in the backend isn't shown and if i inspect the variation_data object there is no key in the array which says "_text_field". How can i store/update variation_data object?
I hope it's understandable which issue i'm facing here and look forward to some help :)
Best wishes,
Florian