Translations not showing – custom plugin

I have a plugin that inserts text into WooCommerce Order Completed email. This works.

I recently i18n’d the code and a colleague used Loco Translate to translate the strings into Dutch and German. I added Dutch and German as WordPress site languages and I am currently running it in Dutch. Unfortunately the translations do not show up when I test my code.

Code: https://gist.github.com/damiencarbery/6d7a0a26cf31d1532e131def134bfc5e

Screenshot: https://snipboard.io/fGWpYA.jpg (I am using Preview Emails plugin; it is translated)

When I look at the global $i10n variable I see that the translated file has been correctly found but the ‘entries’ element is empty:

    '/wp-content/languages/loco/plugins/tracking-info-to-wc-order-nl_NL.l10n.php' => 
    array (
      'nl_NL' => 
      array (
        'tracking-info-to-wc-order' => 
        WP_Translation_File_PHP::__set_state(array(
           'headers' => 
          array (
          ),
           'parsed' => false,
           'error' => NULL,
           'file' => '/wp-content/languages/loco/plugins/tracking-info-to-wc-order-nl_NL.l10n.php',
           'entries' => 
          array (
          ),
           'plural_forms' => NULL,
        )),
      ),
    ),

How should I go about debugging this?

I have also converted the Loco Translate translations into PO and MO files and put them into my wp-content/plugins/tracking-info-to-wc-order/languages directory but that did not fix the issue. (I deleted those files before taking the above screenshot and $i10n data).