I am not 100% sure about this but I think roads on maps are basically there in two ways.
first there is what meets the eye....the road itself
second there is some coded stuff that gives AI units the references of where the roads are on the map, so they can follow them (as AI doesnt use its eyes to locate them)
So when you say you have this problem on 3rd party maps, then it might just be a road placed on it yet not coded for AI use.
Once again, I could be wrong on this one. dont know if things like WrpEdit automatically have the AI road recognition thingie going on when roads are created.