Operating system: Win10 64 bit, April Update
Graphics card: GeForce GTX 1060 6GB.
Broken: Blender 2.8 hash a5869c367e66
Short description of error
In scenes using Branched Path Tracing, areas containing volumetric shading can cause the denoiser to produce very poor results. This occurs with settings allowing 1 or more volume bounces and gets markedly worse if you raise the number of volume samples per AA sample. You can see the reason in the denoising passes, the normal pass flattens while getting artifacts and the albedo pass brightens and gets washed out.
Exact steps for others to reproduce the error
Open the .blend and render. Look at the denoising passes to note the marked decrease in quality where the cube is.