I have problems with shadows when using lumen

I’m very interested in lumens, today I happened to be using the Downtown West Modular Pack to explore lumens. When I put a plane using transparent material the leaves go up along the z axis. It’s still good at first, when the leaf is lifted high, the shadow becomes darker, it is not until the whole plane in the z-axis exceeds the length of the plane that the shadow begins to fade. Please consider.