diff --git a/CRM/Core/Invoke.php b/CRM/Core/Invoke.php
index 1255fd328595..c2d8d5ac7162 100644
--- a/CRM/Core/Invoke.php
+++ b/CRM/Core/Invoke.php
@@ -379,6 +379,10 @@ public static function rebuildMenuAndCaches(bool $triggerRebuild = FALSE, bool $
     $config = CRM_Core_Config::singleton();
     $config->clearModuleList();
 
+    // dev/core#3660 - Activate any new classloaders/mixins/etc before re-hydrating any data-structures.
+    CRM_Extension_System::singleton()->getClassLoader()->refresh();
+    CRM_Extension_System::singleton()->getMixinLoader()->run(TRUE);
+
     // also cleanup all caches
     $config->cleanupCaches($sessionReset || CRM_Utils_Request::retrieve('sessionReset', 'Boolean', CRM_Core_DAO::$_nullObject, FALSE, 0, 'GET'));