How to update Order meta data, from a WooCommerce Action - woocommerce

Question

I'm trying to add some custom meta_data to a WooCommerce Order, by running a Order action.
Here is my code:
function custom_add_order_actions( $actions ){
global $theorder;
$actions['my_custom_action'] = 'My custom action';
return $actions;
}
add_action( 'woocommerce_order_actions', 'custom_add_order_actions' );
function custom_add_single_action( $order ){
// Non of these change anything on the order
$order->set_billing_first_name( 'A new test name' );
$order->update_post_meta( 'a_test_field', 'Test field value' );
update_post_meta( $order->get_id(), 'a_test_field', 'Some other value' );
// $order->save(); // I even tried adding this as well, but it doesn't change anything.
}
add_action( 'woocommerce_order_action_my_custom_action', 'custom_add_single_action' );
How do I change the order (or specifically, post_meta fields for an order) from inside an action?
A example
Imagine that I add a post_meta field, with the field name (key): a_test_field.
It's currently an ACF-field, but it's the same for regular WordPress custom fields.
If I change the value of the field and press 'Update', then the value changes:
So far so good. Now the value of the field is 'Foobar'.
What's wierd is that even if I do this:
add_action( 'woocommerce_order_action_my_custom_action', 'custom_add_single_action' );
function custom_add_single_action( $order ){
update_post_meta( $order->get_id(), 'a_test_field', 'A new value' );
die(); // This die is vital, to make the change in the database.
}
Then I can see the value change in the database to 'A new value'.
But if I just do this:
add_action( 'woocommerce_order_action_my_custom_action', 'custom_add_single_action' );
function custom_add_single_action( $order ){
$order->update_post_meta( 'a_test_field', 'A new value' );
// No die(); here...
}
Then the value remains 'Foobar' in the database.

Answer 1

Sorry but the following lightly revisited code works (Selecting the action and click on the button arrow):
add_action( 'woocommerce_order_actions', 'add_custom_order_action' );
function add_custom_order_action( $actions ){
$actions['my_custom_action'] = __('My custom action', 'WooCommerce');
return $actions;
}
add_action( 'woocommerce_order_action_my_custom_action', 'triggered_custom_order_action' );
function triggered_custom_order_action( $order ){
$order->update_meta_data( '_test_1_custom_field', 'AAFFBB9977' );
$order->save();
update_post_meta( $order->get_id(), '_test_2_custom_field', 'Some other value' );
}
Code goes in functions.php file of the active child theme (or active theme). Tested and works.
Note: Order actions are mostly used for some other things than what you are trying to do.
Now when using a meta box with an input field (as you are showing), on submit, you should save that field value using the action hook save_post_shop_order like in those related threads:
Metabox with multi checkbox in WooCommerce admin single orders
Dynamic custom order numbers based on payment method
Metabox with multiple custom fields for WooCommerce admin order pages