I've tested exactly the same scenario with the newest Sitefinity version and it seems to be working properly. I set a root page o be from the default language of the website (in my case Nelgish, you case -Arabic). Went to an Arabic page and was redirected to the Arabic version of the root page (as expected). There must be something specific about your scenario. I would advise you to open a support ticket and provide us with backend credentislas in it, so that we can observe your rule and siteMap in detail.