diff --git a/tests/system/Session/SessionTest.php b/tests/system/Session/SessionTest.php
index 518a2d733a69..7a8de643cc35 100644
--- a/tests/system/Session/SessionTest.php
+++ b/tests/system/Session/SessionTest.php
@@ -344,6 +344,20 @@ public function testCanFlashData(): void
         $this->assertFalse($session->has('foo'));
     }
 
+    /**
+     * @see https://github.com/codeigniter4/CodeIgniter4/pull/9535#discussion_r2052022296
+     */
+    public function testMarkAsFlashdataFailsWhenAtLeastOneKeyIsNotInSession(): void
+    {
+        $session = $this->getInstance();
+        $session->start();
+
+        $session->set(['foo1' => 'bar1', 'foo2' => 'bar2']);
+
+        $this->assertFalse($session->markAsFlashdata(['foo1', 'foo2', 'foo3']));
+        $this->assertArrayNotHasKey('__ci_vars', $_SESSION);
+    }
+
     public function testCanFlashArray(): void
     {
         $session = $this->getInstance();
@@ -462,6 +476,20 @@ public function testSetTempDataArraySingleTTL(): void
         $this->assertLessThanOrEqual($_SESSION['__ci_vars']['baz'], $time + 200);
     }
 
+    /**
+     * @see https://github.com/codeigniter4/CodeIgniter4/pull/9536#discussion_r2051798869
+     */
+    public function testMarkAsTempdataFailsWhenAtLeastOneKeyIsNotInSession(): void
+    {
+        $session = $this->getInstance();
+        $session->start();
+
+        $session->set(['foo1' => 'bar1', 'foo2' => 'bar2']);
+
+        $this->assertFalse($session->markAsTempdata(['foo1', 'foo2', 'foo3'], 200));
+        $this->assertArrayNotHasKey('__ci_vars', $_SESSION);
+    }
+
     public function testGetTestDataReturnsAll(): void
     {
         $session = $this->getInstance();