I usually experience this bug after a few hours (most of the time appears after demolishing old lines).
More and more resources are being imported via invisible lines, and from the other side of the factory very far away.
I made a video (I think it’s easier to understand it): youtu.be/7c1yVJcsV-A
I also think this bug has to do something with the deleted item reappearing bug. If you demolish a largely built factory, save and load again, you can see lot of the overhead importer lines reappear.