AsyncOperation.allowSceneActivation seems to be ignored

Hi all.

I try to preload a main menu scene while playing an intro, and then show that menu when intro is skipped or finished. All posts that I've seen state that if you load scene async with:

 op.allowSceneActivation = false;    

then op.progress will, hmm... progress up to 0.9f and op.isDone will be false. Then I can manually set allowSceneActivation to true when I want to, and scene will appear. So I wrote this:

 private AsyncOperation op;
 
 void Awake()
 {
     op = SceneManager.LoadSceneAsync(Name, LoadSceneMode.Single);
     op.allowSceneActivation = false;
 }
 
 void Update()
 {
     if (op.progress >= 0.9f && m_Proceed)
     {
         if (!op.allowSceneActivation)
         {
             op.allowSceneActivation = true;
         }
         else if (op.isDone)
         {
             // Switch to main menu processing
         }
     }
 }

But the first time Update is called my async operation has progress = 1f and isDone = true whilst allowSceneActivation = false. My menu comes on screen though I didn't allow it. It does work this way in Single and Additive loading modes. I tried free versions of Unity 5.5.0f3 and 5.5.1p3 and both have the issue.

Am I missing something? How can I preload scene without showing it up on the screen until I want to?

Thanks.