Just a cronological breakdown of the MIS branched pathtracer problem, since it has gotten worse with recent builds.
File: barbershop_interior_cpu.blend from https://www.blender.org/download/demo-files/
Settings: Branched Pathtracing (8AA Samples)² All subsamples:1 Sample All Lights: 1 Sample Disabled Spacial Splits MIS ON
Blender 2.79b ( last working official release before the error occured):
Blender 2.80:
https://imgur.com/a/6500Gdw
Blender 2.81 Alpha 05.10.2019:
As seen, objects now also randomly change their diffuse color when MIS is enabled.
Edit: The change in color in the 2.81 build is unrelated to MIS. This happens by simply switching to branched.