Hi KwangGan,
I've just tried your code and I can't replicate the bug. Here is my video:
http://recordit.co/pAAeG3HXQs
Can you please check again?
PS: I found a small bug in your code: the group title should be defined like this:
'group_title' => array ( 'field' => '_product_file_man_name' )
e.g., do not miss the field
key.