Translatable strings in mimemail.html.twig not available for translation

On my Drupal 11.1.6 website, I have all of the multilingual modules installed. The site's default language is English (en), and there's a secondary language set up: Español (es)

I added a translatable string in my theme's mimemail.html.twig such as:

{{ 'Hello world' | t }}

I registered on the site at http://example.com/es/user/register and the user registration email was sent successfully in Español.

However, my string is unavailable for translation at /admin/config/regional/translate

I went so far as to put my string in html.html.twig, render it, then I found and translated by string at /admin/config/regional/translate - but it still doesn't send in Español in the user registration email.

I'm at a total loss here, so I thought I'd ask here to see if anyone has any recommendations... or if this is a bug with the mimemail module?

I'm using SMTP Mailer as the sender.