['#attributes']['class'] variable is not being rendered on the page

I'm working on an old Drupal 7 site.

In the template file paragraphs-item--user-block-paragraph.tpl.php the content is being rendered, but the class is not added to the element.

Here is the code:

$content['#attributes']['class'][] = 'my-class';
print "<script>alert('".$content['#attributes']['class'][0]."');</script>"; // confirmed variable is set to my-class
print render($content); // confirmed correct file name

What variable in $content determines the classes of the element?

I don't know the structure of the template but it should be something like this:

$content['field'][0]['#attributes']['class'][] = 'my-class';

or

$content['field']['#attributes']['class'][] = 'your-class';