Here is a simple scene. Hires ball and ground (construct) only default textures. Some times it had a white light. Sometimes it doesn't.

Here are the ball, a light, and the ground.

Here it had the ball and one light no ground.

here are no lights with and without the ground respectively.

And here is the scene with raven's setup with and without the ground.

As you can see, the images are the same. It looks like you have the cast_light option checked on the background texture. I get similar to what you get without any lights if I have that checked.

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