I think, it is ab bug of Navigator. There are multiple lanes in each direction separated, if you turn to the left or to to the right. If you want to turn left, you have to cross the first lanes going to the right. Navigator interpretes this as going straight across these lanes. But if you go straight on the lanes marked straight, you are not allowed any more to turn left after crossing the first lanes.
Oops, at this crossing I mixed up the streets. But the problem remains the same. turn lanes are only calculated from road segment to road segment. So the right advice is only given when the intersection is already entered.