Hue Attribute Controller Device (bug?)

OK - I've added an attribute controller device to control the *hue* attribute of one of my color/ambiance bulbs.

But every time I try to use it to change the color setting of my bulb, the brightness setting is also changing to either 0% or 100% depending on whether the bulb in question is dim or bright at the time.

Can I get this thing to manipulate the color only and leave the brightness alone?