Forward tilt at height should not be used as can be seen in this picture. The line projected downwards from the centre of the load shows just how far the load as effectively moved out away from the fulcrum of the truck. The more tilt there is available to the operator and the higher the truck stacks, the worse the problem becomes.
Forward tilt should not be used at height particularly with a laden truck. Forward tilt is meant to assist the operator in withdrawing the forks from a load after the load has been stacked. Suppose that, when trying to withdraw the forks, the front wheels of the truck hit an obstacle then forward tilt can be used as no-load is then on the forks and there is no stability problem.