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.
Because, yes…
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!