Update to Drupal 8.1.1. For more information, see https://www.drupal.org/node/2718713

core/modules/help/src/Controller/HelpController.php

@@ -114,7 +114,7 @@ class HelpController extends ControllerBase {
   public function helpPage($name) {
     $build = array();
     if ($this->moduleHandler()->implementsHook($name, 'help')) {
-      $module_name =  $this->moduleHandler()->getName($name);
+      $module_name = $this->moduleHandler()->getName($name);
       $build['#title'] = $module_name;
 
       $info = system_get_info('module', $name);

core/modules/help/src/Plugin/Block/HelpBlock.php

@@ -88,14 +88,19 @@ class HelpBlock extends BlockBase implements ContainerFactoryPluginInterface {
       return [];
     }
 
-    $help = $this->moduleHandler->invokeAll('help', array($this->routeMatch->getRouteName(), $this->routeMatch));
+    $implementations = $this->moduleHandler->getImplementations('help');
     $build = [];
-
-    // Remove any empty strings from $help.
-    foreach (array_filter($help) as $item) {
-      // Convert strings to #markup render arrays so that they will XSS admin
-      // filtered.
-      $build[] = is_array($item) ? $item : ['#markup' => $item];
+    $args = [
+      $this->routeMatch->getRouteName(),
+      $this->routeMatch,
+    ];
+    foreach ($implementations as $module) {
+      // Don't add empty strings to $build array.
+      if ($help = $this->moduleHandler->invoke($module, 'help', $args)) {
+        // Convert strings to #markup render arrays so that they will XSS admin
+        // filtered.
+        $build[] = is_array($help) ? $help : ['#markup' => $help];
+      }
     }
     return $build;
   }

core/modules/help/src/Tests/HelpBlockTest.php

@@ -14,7 +14,7 @@ class HelpBlockTest extends WebTestBase {
   /**
    * {@inheritdoc}
    */
-  public static $modules = ['help', 'help_page_test', 'block'];
+  public static $modules = ['help', 'help_page_test', 'block', 'more_help_page_test'];
 
   /**
    * The help block instance.
@@ -39,6 +39,12 @@ class HelpBlockTest extends WebTestBase {
     $this->drupalGet('help_page_test/no_help');
     // The help block should not appear when there is no help.
     $this->assertNoText($this->helpBlock->label());
+
+    // Ensure that if two hook_help() implementations both return a render array
+    // the output is as expected.
+    $this->drupalGet('help_page_test/test_array');
+    $this->assertText('Help text from more_help_page_test_help module.');
+    $this->assertText('Help text from help_page_test_help module.');
   }
 
 }

core/modules/help/src/Tests/HelpTest.php

@@ -51,12 +51,12 @@ class HelpTest extends WebTestBase {
    * Logs in users, tests help pages.
    */
   public function testHelp() {
-    // Login the root user to ensure as many admin links appear as possible on
+    // Log in the root user to ensure as many admin links appear as possible on
     // the module overview pages.
     $this->drupalLogin($this->rootUser);
     $this->verifyHelp();
 
-    // Login the regular user.
+    // Log in the regular user.
     $this->drupalLogin($this->anyUser);
     $this->verifyHelp(403);
 

core/modules/help/tests/modules/help_page_test/help_page_test.module

@@ -19,6 +19,8 @@ function help_page_test_help($route_name, RouteMatchInterface $route_match) {
       return t('Read the <a href=":url">online documentation for the Help Page Test module</a>.', [':url' => 'http://www.example.com']);
     case 'help_page_test.has_help':
       return t('I have help!');
+    case 'help_page_test.test_array':
+      return ['#markup' => 'Help text from help_page_test_help module.'];
   }
 
   // Ensure that hook_help() can return an empty string and not cause the block

core/modules/help/tests/modules/help_page_test/help_page_test.routing.yml

@@ -11,3 +11,10 @@ help_page_test.no_help:
     _controller: '\Drupal\help_page_test\HelpPageTestController::noHelp'
   requirements:
     _access: 'TRUE'
+
+help_page_test.test_array:
+  path: '/help_page_test/test_array'
+  defaults:
+    _controller: '\Drupal\help_page_test\HelpPageTestController::testArray'
+  requirements:
+    _access: 'TRUE'

core/modules/help/tests/modules/help_page_test/src/HelpPageTestController.php

@@ -27,4 +27,14 @@ class HelpPageTestController {
     return ['#markup' => 'A route without help.'];
   }
 
+  /**
+   * Provides a route which has multiple array returns from hook_help().
+   *
+   * @return array
+   *   A render array.
+   */
+  public function testArray() {
+    return ['#markup' => 'A route which has multiple array returns from hook_help().'];
+  }
+
 }

core/modules/help/tests/modules/more_help_page_test/more_help_page_test.info.yml

@@ -0,0 +1,7 @@
+name: 'More Help Page Test'
+type: module
+description: 'Module to test the help page.'
+package: Testing
+version: VERSION
+core: 8.x
+hidden: true

core/modules/help/tests/modules/more_help_page_test/more_help_page_test.module

@@ -0,0 +1,20 @@
+<?php
+
+/**
+ * @file
+ * More Help Page Test module to test the help blocks.
+ */
+
+use \Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Implements hook_help().
+ */
+function more_help_page_test_help($route_name, RouteMatchInterface $route_match) {
+
+  switch ($route_name) {
+    // Return help for the same route as the help_page_test module.
+    case 'help_page_test.test_array':
+      return ['#markup' => 'Help text from more_help_page_test_help module.'];
+  }
+}