It looks like the effect is only this:
When someone points a flashlight at a mirror, and it hits your eyes, then the light blinds you very bright. In UE the material that would be the mirror becomes then very bright, and it can happen that you can not see the material anymore, because the light is blinding you.
I didn’t think initally that it is this effect, because this effect would normally never be so strong, especially when you look at a table or at a normal wall. The effect also explains why the material can change from very dark to very bright only by changing the camera 1 meter to the side, it can look like a bug, and look very strange. It’s also still a mystery how it works in darkness, because when you go closer to a wall in darkness, then there is always a flashlight-like reflection, no matter where you put your light. This seems unnatural, and it looks like a camera-effect and not like a reflection, but I think it’s the same reflection. I think there are more options about it, and with more knowledge and fine-tuning you could make it very realistic. The only suggestion here was to turn down specular, but I wouldn’t recommend it as the only solution, I think there are much more options you could use to make it look better.