It's not alleged, it's actual. Just reach out to their support or book a genius bar appointment and they'll tell you to do everything you've already done and then insist that it's not their phone or iOS 11 and that it's a Mercedes issue and that the dealer will need to do a firmware update on your infotainment system to make your car compatible with their phone, which won't fix the issue.
My BT wouldn't stay connected to my 2015 Nissan Murano that I recently traded in and it won't stay connected to my 2016 Acura TLX that replaced my Nissan and it won't stay connected to my Wife's 2012 Nissan Murano. My wife's iPhone 6 however has no issue staying to connected to her car or my Acura. I'm beyond frustrated.