Uhm, that not a bug, it's a feature, and a totally correct one )) The idea is that the city wall should face outside the city. And yes, this property is actually present for all walls and fences, just fewer renderers show them as having different sides. But, for example, if you export any part of a map with fences into OpenOrienteering Mapper, you'll see that regular fences can either have ticks inside an enclosure (correct situation) or outside them. But that's too rare to be cared about...
Here OM shows everything correctly, so the wall should indeed be reversed.