What is happening with my landscape? (fresh project)

I made a fresh new project in 4.18.
Generated a pretty large, but generic landscape in my level.
Put down a skylight and the default skybox from the engine content.
Now I have an effect as if my frustum culls everything except the forward side of the camera view cube. No matter where I look, it is only this part of the camera view that I can see my scene in. **See the first image: The black part of the viewport is from looking slightly downward with my camera. **If I looked downward completely, it would be black, while if I looked up, it would show me the skybox as is.
I played around in another level with the effects where it worked the other way around - the square that previously was the only coordinate system side for viewing the scene now was the darker part and the other 8 sides showed scene normally at first, HOWEVER, after I cranked up my skybox star intensity, they all showed onto this screen-space projected square. See the second image: the same forward facing side of the frustum. **ALSO this effect was only present **when I set a material that is reflective onto the landscape/mesh.
So that led me to a theory that it is some sort of a screen-space reflection effect originating from the underlying physically based lighting system.
If all is indeed working as intended, can I make the reflectivity on the material have gradient borders? Cause the specular angle does not work like that in real life, I think and the cut-off in the frustum is very confusing.
I would be thankful for any elaborations/corrections.