Axis Aligned Sky Reflection "Cube"

Hi everyone !
I have a “quality” problem when rendering some cinematics in my project: one can see a “line” splitting quality settings which is quite noticeable depending on the view point. I know how to tweak LOD distances, CSM distance and blending but my concern is about reflections, specifically sky reflections.

For glossy/specular materials, it looks like the engine is using an axis-aligned cube centered around the camera. All objects within this volume gets nice reflections color from the sky but problem is that boundary is harsh and one could see it in the final image… Cf screenshot below:

So my questions are:

  • How can I extend the size of the box for my cinematics rendering?
  • Or how can I make sky reflections intensity fade to make this hard edge disappear?

Thanks for your help!