I am currently working on a canyon scene with quixel megascans and i just noticed this small glitch or bug in cycles.
and for more clear proof here it is: I add a cube and completely blocked the light source.
Make sure that you have a proper material setup. In your Image Texture Nodes, only Albedo/Diffuse map should have a color space set to Color, all others are non-color data. In this case Emission will be 0. Also, it’s not recommended to have normals and displacements with values more than 1. And keep an eye to object dimensions, this will get you more realistic results and avoid potencial lighting issues.
