Consider a system on the level ground (let this be the datum line). Only gravitational force(force 1) and a vertically upward force(force 2) are acting on it and there is no heat transfer. When the upward force, force 2, is greater than gravitational force, force 1, there is a net-upward force which causes the system to accelerate up. After a certain distance, force 1 and force 2 become equal and the system starts moving at a constant velocity. The net-upward force multiplied by the distance traveled gives the work done on the system by the net-force which is equal to the increase in kinetic and potential energy of the system during acceleration.
Now, when it is moving at a constant velocity, as there is no net force acting on the system, the net-work would be zero. But still, the potential energy is increasing with the kinetic energy remaining constant. How is the energy in the system increasing without any work being done on it?