As you can see on below screenshot all metallic surfaces are black. It happen with either my own cubemap or the stock ones. There is single post process volume (unbound).
It also happen when light sources are active (but to less degree, only surface with metallnes on 1 and very low roughness do not pick up cubemap).
I guess it happen due to lack information of indirect lighting, since there is none. As in that particular scene lighting can’t bounce from anything (as there is only single object - gun).