If I were to guess (and I’m about to) I’d say your fix32 is off and that you aren’t really getting negative values back from the device.
I’m guessing this because I had the same trouble and it was an error on my part.
For a test – run your code against a Kodak scanning in simulation mode as I am certain that they don’t return negative units.
But as to your question, yes. if you set the units to xyx you should be getting responses for pretty much everything in xyx units except where noted otherwise in the spec. There are a couple of triplets that are documented as always returning in one unit or another – no I don’t remember exactly which ones (*should) always return in a certain unit but it’s in the spec.
.