tests/src/Functional/CommentTest.php      | 22 ++++++++++++++++++----
 tests/src/Functional/NodeTest.php         |  3 +--
 tests/src/Functional/ResourceTestBase.php |  7 -------
 3 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/tests/src/Functional/CommentTest.php b/tests/src/Functional/CommentTest.php
index 0b9781a..e0eed4c 100644
--- a/tests/src/Functional/CommentTest.php
+++ b/tests/src/Functional/CommentTest.php
@@ -46,10 +46,6 @@ class CommentTest extends ResourceTestBase {
    */
   protected static $patchProtectedFieldNames = [
     'status' => "The 'administer comments' permission is required.",
-    // @todo These are relationships, and cannot be tested in the same way. Fix in https://www.drupal.org/project/jsonapi/issues/2939810.
-    // 'pid' => NULL,
-    // 'entity_id' => NULL,
-    // 'uid' => NULL,
     'name' => "The 'administer comments' permission is required.",
     'homepage' => "The 'administer comments' permission is required.",
     'created' => "The 'administer comments' permission is required.",
@@ -57,6 +53,10 @@ class CommentTest extends ResourceTestBase {
     'thread' => NULL,
     'entity_type' => NULL,
     'field_name' => NULL,
+    // @todo Uncomment this after https://www.drupal.org/project/drupal/issues/1847608 lands. Until then, it's impossible to test this.
+    // 'pid' => NULL,
+    'entity_id' => NULL,
+    'uid' => "The 'administer comments' permission is required.",
   ];
 
   /**
@@ -459,4 +459,18 @@ class CommentTest extends ResourceTestBase {
     return $cacheability;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function testPatchIndividual() {
+    // Ensure ::getModifiedEntityForPatchTesting() can pick an alternative value
+    // for the 'entity_id' field.
+    EntityTest::create([
+      'name' => $this->randomString(),
+      'type' => 'bar',
+    ])->save();
+
+    return parent::testPatchIndividual();
+  }
+
 }
diff --git a/tests/src/Functional/NodeTest.php b/tests/src/Functional/NodeTest.php
index 21074dd..6e2d2ce 100644
--- a/tests/src/Functional/NodeTest.php
+++ b/tests/src/Functional/NodeTest.php
@@ -49,13 +49,12 @@ class NodeTest extends ResourceTestBase {
    */
   protected static $patchProtectedFieldNames = [
     'revision_timestamp' => NULL,
-    // @todo This is a relationship, and cannot be tested in the same way. Fix in https://www.drupal.org/project/jsonapi/issues/2939810.
-    // 'revision_uid' => NULL,
     'created' => "The 'administer nodes' permission is required.",
     'changed' => NULL,
     'promote' => "The 'administer nodes' permission is required.",
     'sticky' => "The 'administer nodes' permission is required.",
     'path' => "The following permissions are required: 'create url aliases' OR 'administer url aliases'.",
+    'revision_uid' => NULL,
   ];
 
   /**
diff --git a/tests/src/Functional/ResourceTestBase.php b/tests/src/Functional/ResourceTestBase.php
index 068eecf..c10a454 100644
--- a/tests/src/Functional/ResourceTestBase.php
+++ b/tests/src/Functional/ResourceTestBase.php
@@ -2325,13 +2325,6 @@ abstract class ResourceTestBase extends BrowserTestBase {
       $field = $modified_entity->get($field_name);
       $original_values[$field_name] = $field->getValue();
       switch ($field->getItemDefinition()->getClass()) {
-        case EntityReferenceItem::class:
-          // EntityReferenceItem::generateSampleValue() picks one of the last 50
-          // entities of the supported type & bundle. We don't care if the value
-          // is valid, we only care that it's different.
-          $field->setValue(['target_id' => 99999]);
-          break;
-
         case BooleanItem::class:
           // BooleanItem::generateSampleValue() picks either 0 or 1. So a 50%
           // chance of not picking a different value.