I don't know how to solve this. I guess somehow even though I put a plane in the background, the depth of my Z plane is too much and its blacking out way before the furthest back pixol. I tried changing the Displace Z in Layers, recombined with more planes.. Nothing.
I'm throwing up the zbush Doc so you can see Im not crazy *well more than usual*.
I have seen other discussions on here and Zbrush Central with no clear consistent way to overcome this or keep it from happening other than having a plane in the 2.5D pixol canvas document when you start (which I did).