yes, modify ```getActiveSubclass``` like :

```php
public function getActiveSubClass()
    {
        if (!$this->hasActiveSubClass()) {
            $subject = $this->getSubject();

            if($subject) {
                return get_class($subject);
            }

            return null;
        }

        $subClass = $this->getRequest()->query->get('subclass');

        return $this->getSubClass($subClass);
    }
```
and change the call to ```getNewFieldDescriptionInstance``` with :

```php
$fieldDescription = $this->admin->getModelManager()->getNewFieldDescriptionInstance(
-            $this->admin->getClass(),
+            $this->admin->getActiveSubClass() ?: $this->admin->getClass(),
             $name instanceof FormBuilder ? $name->getName() : $name,
             $fieldDescriptionOptions
         ); 
```

Tests passed and my demo is ok, but i don't know if they are "side effects"...