diff -u b/src/Entity/MediaBundle.php b/src/Entity/MediaBundle.php
--- b/src/Entity/MediaBundle.php
+++ b/src/Entity/MediaBundle.php
@@ -250,12 +250,17 @@
   public function preSave(EntityStorageInterface $storage) {
     parent::preSave($storage);
 
+    // If the type plugin uses a source field, we'll need to store its name in
+    // our type configuration before saving. We'd need to double-save if we did
+    // this in postSave().
     $type_plugin = $this->getType();
     if ($type_plugin instanceof SourceFieldInterface) {
       $storage = $type_plugin->getSourceField($this)->getFieldStorageDefinition();
+      // If the field storage is a new (unsaved) config entity, save it.
       if ($storage instanceof FieldStorageConfigInterface && $storage->isNew()) {
         $storage->save();
       }
+      // Store the field name.
       $configuration = $type_plugin->getConfiguration();
       $configuration['source_field'] = $storage->getName();
       $this->setTypeConfiguration($configuration);
@@ -268,15 +273,24 @@
   public function postSave(EntityStorageInterface $storage, $update = TRUE) {
     parent::postSave($storage, $update);
 
+    // If the type plugin is using a source field, we may need to save it if
+    // it's new. The field storage is guaranteed to exist already because
+    // preSave() took care of that.
     $type_plugin = $this->getType();
     if ($type_plugin instanceof SourceFieldInterface) {
       $field = $type_plugin->getSourceField($this);
 
+      // If the field is new, save it and add it to this bundle's view and form
+      // displays.
       if ($field->isNew()) {
+        // Ensure the field is saved correctly before adding it to the displays.
+        $field->save();
+
         $entity_type = $field->getTargetEntityTypeId();
         $bundle = $field->getTargetBundle();
 
         if ($field->isDisplayConfigurable('form')) {
+          // Use the default widget and settings.
           $component = \Drupal::service('plugin.manager.field.widget')
             ->prepareConfiguration($field->getType(), []);
 
@@ -285,6 +299,7 @@
             ->save();
         }
         if ($field->isDisplayConfigurable('view')) {
+          // Use the default formatter and settings.
           $component = \Drupal::service('plugin.manager.field.formatter')
             ->prepareConfiguration($field->getType(), []);
 
@@ -292,7 +307,6 @@
             ->setComponent($field->getName(), $component)
             ->save();
         }
-        $field->save();
       }
     }
   }
diff -u b/src/MediaTypeBase.php b/src/MediaTypeBase.php
--- b/src/MediaTypeBase.php
+++ b/src/MediaTypeBase.php
@@ -15,7 +15,7 @@
 /**
  * Base implementation of media type plugin.
  */
-abstract class MediaTypeBase extends PluginBase implements MediaTypeInterface, SourceFieldInterface, ContainerFactoryPluginInterface {
+abstract class MediaTypeBase extends PluginBase implements SourceFieldInterface, ContainerFactoryPluginInterface {
   use StringTranslationTrait;
 
   /**
@@ -143,7 +143,7 @@
     $options = [];
 
     foreach ($this->entityFieldManager->getFieldStorageDefinitions('media') as $field_name => $field) {
-      $allowed_type = in_array($field->getType(), $this->pluginDefinition['allowed_field_types']);
+      $allowed_type = in_array($field->getType(), $this->pluginDefinition['allowed_field_types'], TRUE);
       if ($allowed_type && !$field->isBaseField()) {
         $options[$field_name] = $field->getLabel();
       }
@@ -187,32 +187,24 @@
     if (empty($this->configuration['source_field'])) {
       return $this->createSourceField($bundle);
     }
-    $id = 'media.' . $bundle->id() . '.' . $this->configuration['source_field'];
-
-    return $this->entityTypeManager
-      ->getStorage('field_config')
-      ->load($id)
-      ?:
-      $this->createSourceField($bundle);
+    $field = $this->configuration['source_field'];
+    $fields = $this->entityFieldManager->getFieldDefinitions('media', $bundle->id());
+    return isset($fields[$field]) ? $fields[$field] : $this->createSourceField($bundle);
   }
 
   /**
    * Returns the source field storage definition.
    *
-   * @return \Drupal\field\FieldStorageConfigInterface
+   * @return \Drupal\Core\Field\FieldStorageDefinitionInterface
    *   The field storage definition. Will be unsaved if new.
    */
   protected function getSourceFieldStorage() {
-    if ($this->configuration['source_field']) {
-      $id = 'media.' . $this->configuration['source_field'];
-
-      return $this->entityTypeManager
-        ->getStorage('field_storage_config')
-        ->load($id);
-    }
-    else {
+    if (empty($this->configuration['source_field'])) {
       return $this->createSourceFieldStorage();
     }
+    $field = $this->configuration['source_field'];
+    $fields = $this->entityFieldManager->getFieldStorageDefinitions('media');
+    return isset($fields[$field]) ? $fields[$field] : $this->createSourceFieldStorage();
   }
 
   /**
diff -u b/src/SourceFieldInterface.php b/src/SourceFieldInterface.php
--- b/src/SourceFieldInterface.php
+++ b/src/SourceFieldInterface.php
@@ -5,7 +5,7 @@
 /**
  * Interface for media type plugins that depend on a field.
  */
-interface SourceFieldInterface {
+interface SourceFieldInterface extends MediaTypeInterface {
 
   /**
    * Returns the source field for a bundle using this plugin.
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
@@ -12,7 +12,7 @@
  *   id = "test_type",
  *   label = @Translation("Test type"),
  *   description = @Translation("Test media type."),
- *   allowed_field_types = {"string"}
+ *   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
@@ -1,7 +1,6 @@
 <?php
 
 namespace Drupal\Tests\media_entity\FunctionalJavascript;
-use Drupal\media_entity\Entity\MediaBundle;
 
 /**
  * Tests the media bundle creation.
@@ -66,11 +65,14 @@
    */
   public function testBundleCreationReuseSourceField() {
     // Create a new bundle, which should create a new field we can reuse.
-    MediaBundle::create([
-      'id' => 'pastafazoul',
-      'label' => 'Pastafazoul',
-      'type' => 'generic',
-    ])->save();
+    $this->drupalGet('/admin/structure/media/add');
+    $page = $this->getSession()->getPage();
+    $page->fillField('label', 'Pastafazoul');
+    $this->getSession()
+      ->wait(5000, "jQuery('.machine-name-value').text() === 'pastafazoul'");
+    $page->selectFieldOption('Type provider', 'generic');
+    $this->assertSession()->assertWaitOnAjaxRequest();
+    $page->pressButton('Save media bundle');
 
     $label = 'Bundle reusing Default Field';
     $bundleMachineName = str_replace(' ', '_', strtolower($label));