Clear WP_Object_Cache on WPEngine to get around ACF's updating bug.

functions.php

<?php

// Register script and add necessary options to it, make sure the path is correct
function flush_redis_enqueue_scripts() {
  wp_enqueue_script( 'flush-redis', plugin_dir_url( __FILE__ ) . 'js/script.js', array( 'jquery' ) );
  wp_localize_script( 'flush-redis', 'FlushRedis', array(
    'ajaxUrl' => admin_url( 'admin-ajax.php' ),
    'flushRedisNonce' => wp_create_nonce( 'flush-redis-nonce' ),
    )
  );
}
add_action( 'admin_enqueue_scripts', 'flush_redis_enqueue_scripts' );

// Add the Flush Redis button in the top admin bar
function add_redis_flush_to_admin($admin_bar) {
  $admin_bar->add_menu( array(
    'id'    => 'redis-flush',
    'parent' => false,
    'title' => 'Flush Redis',
    'href'  => '#',
    'meta'  => array(
      'class' => 'flush-redis',
      'title' => __('Flush Redis'),
    ),
  ) );
}
add_action('admin_bar_menu', 'add_redis_flush_to_admin', 100);

// Clear object cache, register function as AJAX
function flush_redis() {
  global $wp_object_cache;

  $nonce = $_POST['nonce'];

  if ( ! wp_verify_nonce( $nonce, 'flush-redis-nonce' ) ) {
    die ( 'Busted!' );
  }

  echo $wp_object_cache->flush();
  exit;
}
add_action( 'wp_ajax_flush_redis', 'flush_redis' );

// Add a Redis flushed notice only when redis-flushed GET parameter is set
function flush_redis_notice() {
  if ( isset( $_GET['redis-flushed'] ) && $_GET['redis-flushed'] == 1 ) {
    echo '<div class="updated"><p><strong>Redis flushed.</strong></p></div>';
  }
}
add_action( 'admin_notices', 'flush_redis_notice' );

script.js

(function($) {
  $(document).ready(function() {
    $('.flush-redis').click(function(event) {
      $.ajax({
        type : "post",
        url : FlushRedis.ajaxUrl,
        data : {action: "flush_redis", nonce: FlushRedis.flushRedisNonce},
        success: function(response) {
          var separator = (document.URL.indexOf('?') === -1) ? "?" : "&";
          window.location = document.URL + separator + "redis-flushed=" + response
        }
      });

      event.preventDefault();
    });
  });
})(jQuery);