I don't know if this is known or intentional but it looks to me like there's a problem with the aero decoupler. When attached to liquid fuel tanks, and a liquid fuel tank/engine is attached to the decoupler, those engines drain the fuel from the tanks to which the decoupler is attached (and the tanks directly on top of that one) first, before draining fuel from the tanks under the decoupler. This means that any liquid engine attached to those first tanks has less fuel to use. None even if it's just one liquid fuel tank.
