diff --git a/core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php b/core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
index 1f5a4c2..01d1b43 100644
--- a/core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
+++ b/core/lib/Drupal/Core/Entity/EntityReferenceSelection/SelectionPluginBase.php
@@ -3,6 +3,7 @@
 namespace Drupal\Core\Entity\EntityReferenceSelection;
 
 use Drupal\Component\Plugin\ConfigurablePluginInterface;
+use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Database\Query\SelectInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\PluginBase;
@@ -84,9 +85,11 @@ public function setConfiguration(array $configuration) {
     $handler_settings = !empty($configuration['handler_settings']) ? $configuration['handler_settings'] : [];
     unset($configuration['handler_settings']);
 
-    // Merge in BC layer and defaults. Settings passed in the root level take
-    // precedence over BC settings.
-    $this->configuration = $configuration + $handler_settings + $default_config;
+    // Settings passed in the root level take precedence over BC settings.
+    $configuration += $handler_settings;
+
+    // Merge in BC layer and defaults.
+    $this->configuration = NestedArray::mergeDeep($default_config, $configuration);
 
     // Synchronize back 'handler_settings'.
     // @todo Remove this sync in Drupal 9.0.x
diff --git a/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
index de6f665..f041727 100644
--- a/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
+++ b/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php
@@ -61,7 +61,6 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
     $form = parent::buildConfigurationForm($form, $form_state);
 
     $configuration = $this->getConfiguration();
-    debug($configuration['sort']);
     $entity_type_id = $configuration['target_type'];
     $entity_type = $this->entityManager->getDefinition($entity_type_id);
     $bundles = $this->entityManager->getBundleInfo($entity_type_id);
diff --git a/core/modules/taxonomy/src/Plugin/EntityReferenceSelection/TermSelection.php b/core/modules/taxonomy/src/Plugin/EntityReferenceSelection/TermSelection.php
index 7a17cf7..75fc052 100644
--- a/core/modules/taxonomy/src/Plugin/EntityReferenceSelection/TermSelection.php
+++ b/core/modules/taxonomy/src/Plugin/EntityReferenceSelection/TermSelection.php
@@ -23,6 +23,18 @@ class TermSelection extends DefaultSelection {
   /**
    * {@inheritdoc}
    */
+  public function defaultConfiguration() {
+    return [
+      'sort' => [
+        'field' => 'name',
+        'direction' => 'asc',
+      ]
+    ] + parent::defaultConfiguration();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
     $form = parent::buildConfigurationForm($form, $form_state);
 
@@ -41,9 +53,6 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
    */
   public function getReferenceableEntities($match = NULL, $match_operator = 'CONTAINS', $limit = 0) {
     if ($match || $limit) {
-      $configuration = $this->getConfiguration();
-      $configuration['sort'] = ['field' => 'name', 'direction' => 'asc'];
-      $this->setConfiguration($configuration);
       return parent::getReferenceableEntities($match, $match_operator, $limit);
     }
 
diff --git a/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php b/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php
index f4ec44e..0ad56b0 100644
--- a/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/EntityReferenceSelection/EntityReferenceSelectionUnitTest.php
@@ -37,14 +37,20 @@ public function providerTestSetConfiguration() {
       [
         [
           'setting1' => 'foo',
-          'setting2' => ['bar', 'baz'],
+          'setting2' => [
+            'bar' => 'bar value',
+            'baz' => 'baz value',
+          ],
         ],
       ],
       [
         [
           'handler_settings' => [
             'setting1' => 'foo',
-            'setting2' => ['bar', 'baz'],
+            'setting2' => [
+              'bar' => 'bar value',
+              'baz' => 'baz value',
+            ],
           ],
         ],
       ],
@@ -52,14 +58,20 @@ public function providerTestSetConfiguration() {
         [
           'setting1' => 'foo',
           'handler_settings' => [
-            'setting2' => ['bar', 'baz'],
+            'setting2' => [
+              'bar' => 'bar value',
+              'baz' => 'baz value',
+            ],
           ]
         ],
       ],
       [
         [
           'setting1' => 'foo',
-          'setting2' => ['bar', 'baz'],
+          'setting2' => [
+            'bar' => 'bar value',
+            'baz' => 'baz value',
+          ],
           'handler_settings' => [
             // Same setting from root level takes precedence.
             'setting2' => 'this will be overwritten',
@@ -80,18 +92,25 @@ public function providerTestSetConfiguration() {
    */
   public function testSetConfiguration($options) {
     $selection = new TestSelection($options, 'test_selector', []);
-    $selection->setConfiguration($options);
 
     $expected = [
       'target_type' => NULL,
       'handler' => 'test_selector',
       'entity' => NULL,
       'setting1' => 'foo',
-      'setting2' => ['bar', 'baz'],
+      'setting2' => [
+        'qux' => 'qux value',
+        'bar' => 'bar value',
+        'baz' => 'baz value',
+      ],
       'setting3' => 'foobar',
       'handler_settings' => [
         'setting1' => 'foo',
-        'setting2' => ['bar', 'baz'],
+        'setting2' => [
+          'qux' => 'qux value',
+          'bar' => 'bar value',
+          'baz' => 'baz value',
+        ],
         'setting3' => 'foobar',
       ],
     ];
@@ -117,11 +136,11 @@ public function testSetConfigurationBcLevel() {
       'handler' => 'test_selector',
       'entity' => NULL,
       'setting1' => 'foo',
-      'setting2' => ['qux'],
+      'setting2' => ['qux' => 'qux value'],
       'setting3' => 'foobar',
       'handler_settings' => [
         'setting1' => 'foo',
-        'setting2' => ['qux'],
+        'setting2' => ['qux'  => 'qux value'],
         'setting3' => 'foobar',
       ],
     ];
@@ -147,7 +166,9 @@ class TestSelection extends SelectionPluginBase {
 
   public function defaultConfiguration() {
     return [
-      'setting2' => ['qux'],
+      'setting2' => [
+        'qux' => 'qux value',
+      ],
       'setting3' => 'foobar',
     ] + parent::defaultConfiguration();
   }