raytracec mirror not reflecting light

Just playing around with DXR and got into a smal problem.
I wanted to use just one directional light as sun and then let the bounces iliuminate everything with RTGI but it just looked to dark.
So i tryed a bit and it looked like the mirrows in my scene canceled some light.
Started a new scene and sadly it’s true. Mirrows actually don’t bounce light at all. You can see the reflections from lit objects and reflections from lit objects in other mirrow like objects…
but if you manipulate the scene to the point that the light has to bounce of the mirrow to illuminate the rest of the room/scene everything stays dark.
Is that a bug in my UE-install or is that supposed to be that way?