commerce – Custom state event subscriber isn’t invoked

I added a new order state called ‘refunded’. I’m trying to have an event get triggered when the order state transitions to ‘refund’, but I can’t get the event to fire.

commerce_order.workflows.yml

order_default:
  id: order_default
  group: commerce_order
  label: 'Default'
  states:
    draft:
      label: Draft
    completed:
      label: Completed
    canceled:
      label: Canceled
    refunded:
      label: Refunded
  transitions:
    place:
      label: 'Place order'
      from: (draft)
      to: completed
    cancel:
      label: 'Cancel order'
      from: (draft)
      to:   canceled
    refund:
      label: 'Refund order'
      from: (completed)
      to: refunded

(...)

This is my module services yml file:

services:
  my_module.order_refund.event_subscriber:
    class: 'Drupalmy_moduleEventSubscriberOrderRefundSubscriber'
    tags:
      - { name: 'event_subscriber' }

And finally the event subscriber class in my_module/src/EventSubscriber/OrderRefundSubscriber.php

<?php

namespace Drupalmy_moduleEventSubscriber;

use SymfonyComponentEventDispatcherEventSubscriberInterface;
use Drupalstate_machineEventWorkflowTransitionEvent;
use DrupalCoreEntityEntityTypeManager;

class OrderRefundSubscriber implements EventSubscriberInterface {
  /**
   * DrupalCoreEntityEntityTypeManager definition.
   *
   * @var DrupalCoreEntityEntityTypeManager
   */
  protected $entityTypeManager;

  /**
   * Constructor.
   */
  public function __construct(EntityTypeManager $entity_type_manager) {
    $this->entityTypeManager = $entity_type_manager;
  }

  /**
   * {@inheritdoc}
   */
  static function getSubscribedEvents() {
    $events('commerce_order.refund.post_transition') = ('orderRefundHandler');
    return $events;
  }

  /**
   * This method is called whenever the commerce_order.refund.post_transition event is
   * dispatched.
   *
   * @param WorkflowTransitionEvent $event
   */
  public function orderRefundHandler(WorkflowTransitionEvent $event) {
    // Do something here ...
  }
}

This is the part of the code that is supposed to trigger the event:

(...)
    $order_state = $order->getState();
    $order_state_transitions = $order_state->getTransitions();
    if (isset($order_state_transitions('refund'))) {
      $order_state->applyTransitionById('refund');
      $order->save();
    }
(...)

Any ideas why my event subscriber code is not firing?

References: https://git.drupalcode.org/project/state_machine/blob/8.x-1.x/README.md