maybe try turning off WPO in your material and then try this CVAR, also i would try switching raytraced shadows enabled and disabled just to be sure its not caused by directional light because it was the culprit for me in 5.5
r.RayTracing.Geometry.InstancedStaticMeshes.EvaluateWPO 1 - ON
r.RayTracing.Geometry.InstancedStaticMeshes.EvaluateWPO 0 - OFF -default