Using static translations for the primary site / site.php being ignored

I’m having trouble outputting static translations for the primary site, and only the primary site.

Translations structure:

• translations
  • en-US
    • site.php

site.php

<?php

return [
  'UNIQUE_KEY_NAME' => `This is the string I'd like to appear.`,
];

Template

{{ 'UNIQUE_KEY_NAME' | t }}

Output

UNIQUE_KEY_NAME

Is there a way to make this pattern work?
The advantage of storing microcopy and site furniture in a single location is that it is much easier to update and share with copywriters.

If there is a second site, then it appears the site.php for that language will be respected, but when there is just one site, the primary site, its corresponding site.php translation file appears to be ignored. Is this intended?

Thanks ?

<?php

return [
  'UNIQUE_KEY_NAME' => 'This should now work',
];