STALL in flattening layers

Hi,

I've just encountered the phenomenon that Acrobat Pro does STALL when two layers in a PDF file are flattened.

The procedure I just executed is just as

https://helpx.adobe.com/acrobat/11/using/pdf-layers.html#merge_or_flatten_layers_acrobat_pro

1. Open the PDF file.
2. Select the pull-down menu as follows:
       View > Show/Hide > Navigation Panes > Layers
3. Select "Flatten Layers" from the options menu.
   
4. Select "Yes" in confirmation.
   

Then, Acrobat Pro does STALL with the following screenshot:

• Product: Adobe Acrobat XI Pro
  • Version: 11.0.22 and 11.0.23
• OS: Windows XP Pro (SP3)
• Reproducibility: YES (100%!!!)

So,

• Do you encounter this phenomenon under your environment with Windows XP or Windows 7 or later?
• How can I request the bug fix for this phenomenon?

# I am willing to upload the PDF file with which I've just encountered this phenomenon if necessary.

Thanks