It does work, but it's very subtle compared to the other modes and not visible with default'ish settings.
To make it visible:
Set contrast to 5, offset to -1 and roughness to 1.
Set noise variation to anything but 0.
Clumping works now, but only within 0-1 range. Negative values seem to be ran through an abs transform as -0.5 gives the same result as 0.5.
Why it's way less strong and obvious than the other variation methods, I have no idea.