Weird... must be a bug but...
all I had to do in order to fix this was to call
UIAccessibilityIsInvertColorsEnabled();
inside the viewDidLoad method of the viewController. After I added that one line of code, now the notifications come through just fine (although there is still one frame of display where it's displayed wrong). Evidently the inverted colors notifications don't get observed at all unless you call this first. Apple's documentation doesn't tell you this.
Definitely one of the weirder things I've come across.