I'm playing with bump maps in the Physical Surface node. There is definitely "clipping" going on with regard to bump maps. If your bump strength  is too much it will clip off the peaks like would happen when recording audio too loud into a digital recorder. You have to lower the amount to below a certain threshold before it stops clipping the bump map and darkening the texture.

W10, Ryzen 5 1600x, 16Gb,RTX2060Super+GTX980, PP11, 11.3.740