Have you seen Andrew's recent video on the Principled BSDF? He manages some similar looking metal results with a Suzanne head. It starts around 29:30 and thereafter. Of course, everything he's doing with the shader in his video is controlled by PBR texture maps, and yours is obviously procedurally controlled by the mix shader values. I could be completely incorrect of course, but I don't think your results are the effect of a bug. It seems the roughness channel is using the mix input node like a procedural greyscale gradient ramp. This should be possible the same way using Cycles with the Principled shader eventually but obviously hasn't been incorporated yet. I'm thinking it's an intentional effect because the same type of control is possible in other physical engines like Octane, where value params on most nodes are treated as basic gradient intensity factors even if no image maps are present.

______________________________________

My Store
My Free Models
My Video Tutorials
My CG Animations
Instagram: @luxxeon3d
Facebook: https://www.facebook.com/luxxeon