diff -u b/src/Entity/MediaBundle.php b/src/Entity/MediaBundle.php
--- b/src/Entity/MediaBundle.php
+++ b/src/Entity/MediaBundle.php
@@ -247,59 +247,54 @@
   /**
    * {@inheritdoc}
    */
-  public function postSave(EntityStorageInterface $storage, $update = TRUE) {
-    parent::postSave($storage, $update);
+  public function preSave(EntityStorageInterface $storage) {
+    parent::preSave($storage);
 
-    if (!$update) {
-      $type_plugin = $this->getType();
+    $type_plugin = $this->getType();
+    if ($type_plugin instanceof SourceFieldInterface) {
+      $storage = $type_plugin->getSourceField($this)->getFieldStorageDefinition();
+      if ($storage instanceof FieldStorageConfigInterface && $storage->isNew()) {
+        $storage->save();
+      }
+      $configuration = $type_plugin->getConfiguration();
+      $configuration['source_field'] = $storage->getName();
+      $this->setTypeConfiguration($configuration);
+    }
+  }
 
-      if ($type_plugin instanceof SourceFieldInterface) {
-        $field = $type_plugin->getSourceField($this);
+  /**
+   * {@inheritdoc}
+   */
+  public function postSave(EntityStorageInterface $storage, $update = TRUE) {
+    parent::postSave($storage, $update);
 
-        $storage = $field->getFieldStorageDefinition();
-        if ($storage instanceof FieldStorageConfigInterface) {
-          $storage->save();
+    $type_plugin = $this->getType();
+    if ($type_plugin instanceof SourceFieldInterface) {
+      $field = $type_plugin->getSourceField($this);
+
+      if ($field->isNew()) {
+        $entity_type = $field->getTargetEntityTypeId();
+        $bundle = $field->getTargetBundle();
+
+        if ($field->isDisplayConfigurable('form')) {
+          $component = \Drupal::service('plugin.manager.field.widget')
+            ->prepareConfiguration($field->getType(), []);
+
+          entity_get_form_display($entity_type, $bundle, 'default')
+            ->setComponent($field->getName(), $component)
+            ->save();
         }
-
-        if ($field->isNew()) {
-          $entity_type = $field->getTargetEntityTypeId();
-          $bundle = $field->getTargetBundle();
-
-          if ($field->isDisplayConfigurable('form')) {
-            $widget = \Drupal::service('plugin.manager.field.widget')
-              ->getInstance([
-                'field_definition' => $field,
-                'form_mode' => 'default',
-                'configuration' => [],
-              ]);
-
-            entity_get_form_display($entity_type, $bundle, 'default')
-              ->setComponent($field->getName(), [
-                'type' => $widget->getPluginId(),
-                'settings' => $widget->getSettings(),
-              ])
-              ->save();
-          }
-          if ($field->isDisplayConfigurable('view')) {
-            $formatter = \Drupal::service('plugin.manager.field.formatter')
-              ->getInstance([
-                'field_definition' => $field,
-                'view_mode' => 'default',
-                'configuration' => [],
-              ]);
-
-            entity_get_display($entity_type, $bundle, 'default')
-              ->setComponent($field->getName(), [
-                'type' => $formatter->getPluginId(),
-                'settings' => $formatter->getSettings(),
-              ])
-              ->save();
-          }
+        if ($field->isDisplayConfigurable('view')) {
+          $component = \Drupal::service('plugin.manager.field.formatter')
+            ->prepareConfiguration($field->getType(), []);
+
+          entity_get_display($entity_type, $bundle, 'default')
+            ->setComponent($field->getName(), $component)
+            ->save();
         }
         $field->save();
       }
     }
-
   }
 
 }
diff -u b/tests/modules/media_entity_test_type/src/Plugin/MediaEntity/Type/TestType.php b/tests/modules/media_entity_test_type/src/Plugin/MediaEntity/Type/TestType.php
--- b/tests/modules/media_entity_test_type/src/Plugin/MediaEntity/Type/TestType.php
+++ b/tests/modules/media_entity_test_type/src/Plugin/MediaEntity/Type/TestType.php
@@ -11,7 +11,8 @@
  * @MediaType(
  *   id = "test_type",
  *   label = @Translation("Test type"),
- *   description = @Translation("Test media type.")
+ *   description = @Translation("Test media type."),
+ *   allowed_field_types = {"string"}
  * )
  */
 class TestType extends Generic {
diff -u b/tests/src/FunctionalJavascript/BundleCreationTest.php b/tests/src/FunctionalJavascript/BundleCreationTest.php
--- b/tests/src/FunctionalJavascript/BundleCreationTest.php
+++ b/tests/src/FunctionalJavascript/BundleCreationTest.php
@@ -91,9 +91,8 @@
     $this->assertSession()->optionExists('Type provider', 'test_type');
     $page->selectFieldOption('Type provider', 'test_type');
     $this->assertSession()->assertWaitOnAjaxRequest();
-
-    // Un-check creation of default source field.
-    $page->selectFieldOption('type_configuration[generic][source_field]', 'field_media_generic');
+    // Select the existing field for re-use.
+    $page->selectFieldOption('type_configuration[test_type][source_field]', 'field_media_generic');
     $page->pressButton('Save media bundle');
 
     // Check that there are not fields created.
only in patch2:
unchanged:
--- a/tests/src/FunctionalJavascript/MediaUiJavascriptTest.php
+++ b/tests/src/FunctionalJavascript/MediaUiJavascriptTest.php
@@ -140,6 +140,11 @@ class MediaUiJavascriptTest extends MediaEntityJavascriptTestBase {
     $this->assertFalse($loaded_bundle->getStatus());
     $this->assertEquals($loaded_bundle->field_map, ['field_1' => 'name']);
 
+    // We need to clear the statically cached field definitions to account for
+    // fields that have been created by API calls in this test, since they exist
+    // in a separate memory space from the web server.
+    $this->container->get('entity_field.manager')->clearCachedFieldDefinitions();
+
     // Test that a media being created with default status to "FALSE" will be
     // created unpublished.
     /** @var \Drupal\media_entity\MediaInterface $unpublished_media */
only in patch2:
unchanged:
--- a/tests/src/Kernel/BasicCreationTest.php
+++ b/tests/src/Kernel/BasicCreationTest.php
@@ -79,7 +79,7 @@ class BasicCreationTest extends KernelTestBase {
     $this->assertEquals($test_bundle->get('label'), 'Test bundle', 'Could not assure the correct bundle label.');
     $this->assertEquals($test_bundle->get('description'), 'Test bundle.', 'Could not assure the correct bundle description.');
     $this->assertEquals($test_bundle->get('type'), 'generic', 'Could not assure the correct bundle plugin type.');
-    $this->assertEquals($test_bundle->get('type_configuration'), [], 'Could not assure the correct plugin configuration.');
+    $this->assertEquals($test_bundle->get('type_configuration'), ['source_field' => 'field_media_generic_1'], 'Could not assure the correct plugin configuration.');
     $this->assertEquals($test_bundle->get('field_map'), [], 'Could not assure the correct field map.');
   }