I think this is expected, since any small differences between colors will be exaggerated with boosted saturation.
Actually disabling denoising might help, because “splotches” like this would not appear then. 
I noticed that too - saturation is super picky about otherwise invisible noise. Maybe it has something to do with in which order or in which colour space saturation is applied?
That definitely looks undesireable.. almost like bug.
Yes i think too what denoise was involved somehow. Also saturation makes darker at shadows as you can see. I was have to change saturation in photoshop.