Caustics question - z height dependent shadow

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Caustics question - z height dependent shadow

Attempting to learn caustics in water. Getting an unexpected shadow line that seems to be dependent on the z height location of the model, meaning as the model is moved below the z-plane the shadows moves to the left covering less of the photon radius. If the model is moved above the z-plane the shadow line moves right covering more of the caustics.

Thanks in advance for any insight.