blender 2.91.2 bevel node doesn't work properly bevel node doesn't work properly in rendered mode with cycles, it darken the edges rather than soften it
You need to lower the radius in the bevel node.
If your cube is 2 units wide and the bevel radius is 1 m, it means the normals are interpolated right to the middle of each face, generating artifacts. Try the default (0.05).
Examples of the effect of the radius field :