Trying to add a color scatter ramp to a volume, I was finding that my volume was always rendering black in the IPR. After some exploring I found that the moment you drop a ramp node (rgb or float) in your shader network, even without even hooking it up to anything, the volume stops rendering correctly in the IPR. There will be a brief flash in the first pass of the render of the expected look but then quickly jumps to black. Rendering to picture viewer has the correct result. I'm currently using C4D R23.008 Build RB323902 and C4DtoA 3.2.1 (Arnold 220.127.116.11).
Basic volume shader renders correctly in both IPR and Picture Viewer:
Adding a ramp_rgb or ramp_float node even without connecting causes issues:
When connecting everything and rendering, Picture Viewer produces expected result:
Ramp node seems to work just fine with standard shader: