This is a known issue and every Mac user of Arabic script (Farsi, Arabic, Urdu etc.) encounter this problem. I'm not sure if it's documented properly because it is not a stand alone issue. It's an unfortunate byproduct of differences between Apple font technology and MS font technology.
But it is widely discussed in forums of users of this script. Here in IRMUG it was discussed repeatedly. I found two previous discussion on this subject in IRMUG:
http://irmug.com/portal/index.php?name= ... opic&t=155
http://irmug.com/portal/index.php?name= ... topic&t=64
BTW if you click on British flag on the top of the site, you go to MEMUG, our English site. It's better to post in English over there because the knowledge can be shared with other Mac users of Arabic script.
... and welcome to our community!