I have been experiencing this issue myself as well. The default B-52 and C-130 included with X-Plane 11 show a black screen when in "2-D Panel Only" mode, but show a proper 2D cockpit when in "Full Screen Simulator" mode. I was testing with X-Plane 11.05, and once I did the update to 11.11, it seems to be working now. Thanks for fixing this!