You gotta optimize your autoloader, yo.

automatic_updates_extensions/tests/src/Kernel/ExtensionUpdateStageTest.php

@@ -100,6 +100,7 @@ class ExtensionUpdateStageTest extends AutomaticUpdatesExtensionsKernelTestBase
       [
         'update',
         '--with-all-dependencies',
+        '--optimize-autoloader',
         'drupal/my_module:9.8.1',
         'drupal/my_dev_module:1.2.0-alpha1@alpha',
       ],