I just tried testing this and I could not reproduce (both with the UI and arcpy). The exported DDP multi-page PDF appears to be using the settings. I added an RGB composit SID file and used the same settings as you. When I next each page in ArcMap and compare to the resulting output PDF, things look the same.
What version/sp are you using?
Arc you exporting from the UI or arcpy?
Thanks,
Jeff