User interface translation: new strings not discovered unless loaded in non-default language

I have a site on Drupal 8.0.2 with a custom theme. For the sake of this description, I will call the .theme file "custom.theme".

In custom.theme, I added a hook_form_FORM_ID_alter() implementation to add some custom header strings to a form. I used the t() function to make these strings translatable.

Example:

  if (!empty($form['field_some_field'])) {
    $form['field_some_field']['#prefix'] = '<h3>' . t('A Custom Header') . '</h3>';
  }

I loaded the form in the site's default language (en) to verify that the form alter worked properly.

Then I went to admin/config/regional/translate so I could translate "A Custom Header" into other languages. I could not find the string.

Ultimately, I could only do so after I loaded the form in a language other than the default language. Once I did so, I could find and translate the string in admin/config/regional/translate.