It's amazing what just a little bit of focus and attention will give you. I turned it on the the garage and moved the headlights from Auto to On. I typically keep the dimmer switch all the way up (without the dome lights), so moved it down to the min. I then started moving the Ambient knob up/down and (at first) noticed nothing. Then my eye caught my white shoes getting brighter and brighter. Ta-Da. After trying a few things, I noticed that it does change no matter the Dimmer setting (other than Domes on). But in mine, it's only footwell lighting, nothing else. The Dimmer controls all of the other interior lighting, including the cupholders.
But for now, mystery solved (for me at least).