@JanO: the algorithm is not picking every possible route, because there is infinite number of cases, but it is trying to find the shortest one using A* algorithm. So long story short, the algorithm stops when it finds the shortest route and make sure that it is the shortest one (any other cases are supposed to be longer).
I can remember such situations on other maps, but only when the map was with ships. The ware was in a very long queue in a port and it was expected that for a very long time it will not be picked by any ship. But removing and adding resources helped much, because the route was destroyed and any other plank/stone/whatever, which was much closer, could be picked. Maybe that was the case?
The map "colonies" is probably the best case for that, because you can produce one type of ware in one part of map and transport it via ships to another place. I remember that some of my mines were almost completely empty, but after some time (15? 30? minutes) they were filled with food.