u/GeassedbyLelouch

Screenshot
I noticed my production of canned food crashed.
Turns out my iron ore stock evaporated, which shouldn't happen since the iron ore production outpaces the consumption.
After investigating further I managed to identify the problem: my iron mine refuses send its output to the nearby warehouse, but instead sends it directly to the furnaces in the nearby industrial cluster. As a result, the internal storage of the iron mine hits 8/8 and then it stops producing. The furnaces all still work at 100% (I guess it's their carriages getting the ore from the mine?) but my iron ore stockpile vanished and thus the canned food production stopped.
I googled the problem but found little information, most of it years old. One proposed solution was to add a warehouse between the mine and the furnaces, which I did. This fixed the problem, the mine started sending ore to the new warehouse, but only temporarily.
As you can see in the screenshot, the warehouse is directly adjacent to the mine and there IS a road (it did work for a while, after all), but now the warehouse is completely idle and my iron is once again completely gone, because my mine again refuses to send its iron to the warehouse, even though they are direct neighbours.
I don't think the relative proximity of warehouses versus furnaces is what's causing this. Relative proximities never caused an issue for any other product either.

Does anyone know what is causing this and how it can be fixed?

Oh and I am not using any mods.

Edit: I forgot to add the math and production bonuses, which can't be seen in the screenshot.
My iron mine has a +40% productivity bonus from the nearby trade union, it's producing 5.6 iron/min.
The foundries have a net bonus of +10% productivity, so both of them consume 4.4 iron/min combined.
I have 2 canneries which both work at a -25% reduced speed, 2 x 60/90 x 0.75 = 1 iron/min
That means I overproduce 0.2 iron/min, but instead my iron stockpile dried up