custom class does not appear on body field from field_formatter_class

When trying to set up a class for the body text field, the custom value is not applied. This is on a zurb_foundation subtheme, although the active zurb_foundation templates/field.html.twig does not seem to be the source of the problem. The element in question has <div{{ attributes }}> on stable theme, and <div{{ attributes.addClass(classes) }}> in equivalent zurb_foundation.

I switched over to Bartik theme and the field formatter class does appear as expected on the body. bartik uses classy/templates/field/field--text-with-summary.html.twig which inherits from field--text.html.twig which only has {% set attributes = attributes.addClass('clearfix', 'text-formatted') %} and then inherits field.html.twig.

it's not clear to me why this would be a problem: <div{{ attributes.addClass(classes) }}> when <div{{ attributes }}> works fine.

Looking deeper at zurb_foundation.theme I have a feeling the problem is around line 114:

  // Add class to a specific fields across content types.
  switch ($variables['element']['#field_name']) {
    case 'body':
      $classes = array('body');
      break;

After all that seems like it would overwrite anything handed into that classes array beforehand and should more properly be

$classes[] = 'body';

similar to line 143 / 147. (Although I can agree it would make sense to remove the text-secondary on 143 #2940608: Remove class "text-secondary" from teasers ) Will report back here if this fixes it.