Kerbal Space Program

The fuel delivery overlay shows clearly there is another way for fuel to pass among the three top tanks. That passage is bi-directional and allows to keep fuel balanced across all those three tanks, therefore negating the effect of the fuel ducts.

The reason for that passage to exist, is because of how you built the vessel. It can be easily noticed the top tanks on each side are the parent parts for the tanks below them (notice how the whole side highlights green when mousing over one of those top tanks, while doesn't for the tanks below). So, those tanks were positioned first, to be attached to the central structure while there's no other part to make them join, they had to be directly in contact with the central top tank side (allowing a radial attachment). Those three tanks stay connected, allowing passage of fuel, even if then the side tanks have been moved outwards with the move tool. It seems you placed decouplers on the sides of the central middle tank, but those decouplers are not joining the side tanks (so, beware, those won't work when staged to detach the side tanks). Also, you put struts around the top tanks like those could be needed to reinforce the structure: useless as well, those tanks are strictly connected and don't need any strut.

Of course all the above comes clear also examining the craft file structure, links show what parts are joined together and their relationship.

Note: Nice job on misleading tech support.