Operating system: Linux Mint 19.2
Graphics card: 1080 Ti
Broken: 2.80, cycles
Short description of error
Diffuse materials glow in corners or small gaps when using Principled Shader. I am not sure it is bug or correct behavior, but issue is gone if color value (V from HSV) is decreased to 0.9, or Diffuse shader is used instead. Decreasing roughness and specular also helps but that will make totally different material.
Exact steps for others to reproduce the error
Here are image and blend file