, thanks for looking into it, but I think your video shows the issue. What you are seeing is from the screenspace reflections reflecting the change to the ground. That’s not the issue. Instead, look at the left face of the cube in your video: that’s the part receiving cubemap reflections and nothing changes there when you move the slider.
If you try with a smooth metallic opaque material, from the same viewpoint, the left side will be the dominant side changing when you slide the toggle, as it is supposed to be.
Let me know if you see what I’m talking about in the video.