Why is the preview renderer showing a #FFFFFF sphere, but the plane as the light source is a grey. Why?
Why? Upload the .blend and people can tell you why.
At least you didn’t crop the screenshot so it shows that the render result is a composite, not the render result from the scene, so it might be because something you did in the compositor.
It’s because of too low clamp direct value in render properties -> sampling. Clamp value of 2-4 or higher is more normal so it doesn’t darken the image too much, and not often needed for direct lighting.
Thank you so much!