Going deeper into this topic I found out that r.RayTracing.Geometry.InstancedStaticMeshes.EvaluateWPO 1 solves the problem, at least partially. In my cases in fact it makes raytraced shadows react to WPO, but the shadows are misplaced, like a lot!
Take a look at my example, have anyone found a way to fix a similar problem?
Animated video showing my case:
