Problems with mixing two principled shaders

Hello all. I am trying to make a heavily rusted, painted metal material by mixing two principled shaders. Instead of nice rusty spots on the paint, I get big black artifacts, see the picture.

This does not happen when I try the same with two diffuse shaders instead of the principled shaders. I wonder if I am doing something wrong, or is it not possible to mix two principled shaders. Simplified demo file attached.

Looking at the node setup, your test scene does exactly what I’d expect it to. Perhaps if you could post an example of what you hope to achieve.