Exclude CSS functionality is broken

On the module settings page, there is an option to "Exclude links inside these CSS selectors." I tried adding "ul.rrssb-buttons", and when I do this, the external link icon disappears from all links on the site, not just the links matching this selector.

I found this problem trying to fix the RRSSB module to work with this one.

function rrssb_extlink_settings_alter(array &$settings) {
  // Add one CSS selector to ignore links that match that.
  $settings['extlink_css_exclude'] .= ', ul.rrssb-buttons a';
}

This code also causes removes the icon from all links on the site, not just the ones matching the selector.

Confirmed on both 1.1 and 1.x-dev. Drupal 8.7.8