The miamaterialx has an artificial specular highlight, seemingly reflecting the light source, even though no light source is “visible”. This can cause unwanted effects, especially when using multiple lights. The specular highlight can be controlled in the shader attributes go to Advanced > Specular Balance. To disable it the value to 0.
In the example image on the left you can clearly see a reflection of the key, fill and even rim light. On the right, I set the “Specular Balance” to 0.