I have got stuck with this problem for very long and never figure it out. Every time I set up a very simple scene with a directional light as the sun and a skysphere actor as the environment, I got the mirror material giving some black reflection of the environment. It looks like the black reflection is about the empty lower hemisphere of the sky sphere. Is there any way to solve that issue?
I need to clarify that the issue seems to only occur with dynamic lighting. Using an HDRi sky sphere still gives the same result.