Place the following code in a custom Drupal (7.x) module to fix the issue described in https://www.drupal.org/node/2915545. Replace MODULE with your custom module's name.

drupalFixMailchimpBlockSignupFormRedirect.module.php

<?php
/**
 * Implements hook_form_mailchimp_signup_subscribe_form_alter().
 */
function MODULE_form_mailchimp_signup_subscribe_form_alter(&$form, &$form_state) {
  $form['#submit'][] = 'MODULE_mailchimp_signup_subscribe_form_redirect';
}

/**
 * Submit callback for mailchimp signup subscribe form.
 */
function MODULE_mailchimp_signup_subscribe_form_redirect($form, &$form_state) {
  if (!isset($form_state['redirect'])) {
    $form_state['redirect'] = drupal_is_front_page() ? '' : current_path();
  }
}