Temando_Shipping module there is the method
In the comment of this method there is this
* Manifest order at Temando platform.
* Observers don't work:
* - `sales_order_save_commit_after` is no longer triggered in guest checkout
* - `sales_order_save_after` does not have related entities (addresses) persisted yet
* Other promising events like `sales_order_place_after`, `checkout_submit_all_after`,
* `sales_model_service_quote_submit_success` are
* - triggered before the order was saved or
* - not triggered at all in multi address checkout or some payment providers'
* custom checkout implementations (paypal express, sagepay, …).
* @param SalesOrderRepositoryInterface $subject
* @param OrderInterface|\Magento\Sales\Model\Order $salesOrder
* @return OrderInterface
I don't have a response to "why?" I'm just stating that it's by design.
As a bonus
checkout_onepage_controller_success_action is not dispatched in admin because that's event is not triggered at all in the backend.
Maybe you can use a plugin similar to what the Temando module uses.
afterSave plugin on the Order repository class.