I just realized that next to smelting furnaces I could usually find a stack of 128 oxydes of the same type of the furnaces, either on the left side or in the back - if the left side blocked. I'm not sure if it's a bug or why they appear (drones trying to deliver them ?).
#Stacks of 128 resources pilled next to smelting furnaces
azure jackal
final ridge
I have seen this happen when I upgrade the furnace to MK2. Or if you change the type of work it is doing from Steel to Aluminum. In either case it dumps all that it has stored next to it.
On the upgrade to MK2, I think that is a bug and it should be able to keep the resource storage. But if you set the setting to allow drones to deliver, they will clean it up if any of your smelters run low on supplies.
azure jackal
I've discovered this issue on a new furnaces line that I'm setting up, it has not been upgraded in any way, and not I've not change the type either (the first image), the only thing is that the entry line have been full for some time while I'm building everything else before upgrades. But it would make sense it tries to dump everything inside for some reasons.