I am wondering if there is an issue with the fuel tank vapor management system.
The idea being that with only 1/8 tank of fuel, you have a 7/8 tank of vapor. If the fuel is being pulled out of the tank, there is 7/8's of a tank of vapor that more easily expand to take the place of the fuel.
With a fuller gas tank, there is less vapor that is able to expand to replace the lost fuel. All other things being equal, the vacuum pressure should be greater (more vacuum, i.e., resistance to fuel leaving the tank) with more fuel in the tank vs vapor.
At least that's my guess. I'm not familiar with the Touareg's specific fuel vapor system, but it can't be that different than any other vehicle.