Issue #3273008: Installing modules with Drush with AutoUpdates enabled shows an error

package_manager/src/PathLocator.php

@@ -47,8 +47,28 @@ class PathLocator {
    *   The absolute path of the vendor directory.
    */
   public function getVendorDirectory(): string {
-    $reflector = new \ReflectionClass(ClassLoader::class);
-    return dirname($reflector->getFileName(), 2);
+    // There may be multiple class loaders at work.
+    // ClassLoader::getRegisteredLoaders() keeps track of them all, indexed by
+    // the path of the vendor directory they load classes from.
+    $loaders = ClassLoader::getRegisteredLoaders();
+
+    // If there's only one class loader, we don't need to search for the right
+    // one.
+    if (count($loaders) === 1) {
+      return key($loaders);
+    }
+
+    // To determine which class loader is the one for Drupal's vendor directory,
+    // look for the loader whose vendor path starts the same way as the path to
+    // this file.
+    foreach (array_keys($loaders) as $path) {
+      if (str_starts_with(__FILE__, dirname($path))) {
+        return $path;
+      }
+    }
+    // If we couldn't find a match, assume that the first registered class
+    // loader is the one we want.
+    return key($loaders);
   }
 
   /**