Tracking and items ordered missing from shipment email in Magento 2.4

I’ve created a module to notify the customer when a custom carrier has been added. I’ve used an observer on the sales_order_shipment_save_after event but the tracking info and items ordered are empty in the shipment email. I’m guessing I need to load the tracking info and order items into the $shipment object?

Example/SendShipmentTracking/Observer/SendShipmentTrackingProcessor.php

<?php

namespace ExampleSendShipmentTrackingObserver;

use ExampleSendShipmentTrackingHelperData;
use MagentoFrameworkEventObserver;
use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkSerializeSerializerJson;
use MagentoSalesModelOrderShipmentNotifier;
use PsrLogLoggerInterface;

class SendShipmentTrackingProcessor implements ObserverInterface
{
    /**
     * @var LoggerInterface
     */
    protected LoggerInterface $logger;

    /**
     * @var Json
     */
    protected Json $jsonHelper;

    /**
     * @var Data
     */
    protected Data $helperData;

    /**
     * @var Notifier
     */
    protected Notifier $shipmentNotifier;

    /**
     * @param LoggerInterface $logger
     * @param Json $jsonHelper
     * @param Data $helperData
     * @param Notifier $shipmentNotifier
     */
    public function __construct(
        LoggerInterface $logger,
        Json $jsonHelper,
        Data $helperData,
        Notifier $shipmentNotifier
    ) {
        $this->logger = $logger;
        $this->jsonHelper = $jsonHelper;
        $this->helperData = $helperData;
        $this->shipmentNotifier = $shipmentNotifier;
    }

    /**
     * @param Observer $observer
     * @return void
     */
    public function execute(Observer $observer)
    {
        if ($this->helperData->isEnabled()) {
            try {
                $shipment = $observer->getEvent()->getShipment();

                if ($shipment && $this->hasShipmentTracking($shipment)) {
                    $order = $shipment->getOrder();
                    $this->logger->debug('shipment data ' . $this->jsonHelper->serialize($shipment->getData()));
                    $this->shipmentNotifier->notify($order, $shipment);
                }
            } catch (Exception $e) {
                $this->logger->error($e->getMessage());
            }
        }
    }

    /**
     * @param $shipment
     * @return bool
     */
    private function hasShipmentTracking($shipment): bool
    {
        $tracksCollection = $shipment->getTracksCollection();
        $trackingNumber = [];

        foreach ($tracksCollection->getItems() as $track) {
            if ($track->isCustom()) {
                $trackTile = $track->getTitle();
                $carrierServices = $this->helperData->getCarriers();

                if (in_array($trackTile, $carrierServices) && $track->getTrackNumber()) {
                    $trackingNumber[] = $track->getTrackNumber();
                }
            }
        }

        return count($trackingNumber) > 0;
    }
}