Once in a while my 2012 981 with PDK used to do the same thing & I don't even have a steep drive. I do however have to reverse in at an angle very carefully edging in & there is a very slight bump on entrance (& I mean slight). I quite often used to experience the very problem you encounter & that meant taking it out of reverse & engaging drive, moving forward a fraction & then re-engaging reverse (a pain with PDK as well). It just stubbornly wouldn't budge otherwise. Sorry, but I never did find any way to avoid it, though it fortunately didn't occur every time. It seemed to be in my case that the little bump on entry sometimes upset the system. Strangely I've yet to experience the same issue with my 718 (982).