Foundation 6.3.1 has an issue when modifying Reveal where it doesn't remove the is-reveal-open class from the body after closing a reveal. You can hook into the close event to fire this. This was fixed in 6.4 but if you can't update and don't want to modify the core Foundation files, you can add this to your page. Requires jQuery obviously.

foundation-remove-class-modal-close.js

 // Fix issue with foundation not closing "is reveal open"
  $(document).on('closed.zf.reveal', function() {
    if ($('body').hasClass('is-reveal-open')) {
      $('body').removeClass('is-reveal-open');
    }
  });