Wonder how come this big one wasn't fixed so far. Forges calculate resources remaining instead of resources used for some reason, and they take the values when you access them, for ALL resources, as starting values, then they deduct what you used and set the resources to the result. That means that, since time doesn't pause in menus (another issue from where I'm standing), if workers are gathering resources when you smelt, you will lose all they'll gather while you have the forge screen open, for ANY resource, so even those that can't be used by forges.