Just ran into this problem again in 2020. Ramps don't show model hatches or any hatches at all, so I think the plan representation of ramps is actually 2D geometry and not 3D. Re-coding this is probably not a high priority at the factory, so they've let it slide. That said, if I can't get a slope off the railing that's on the ramp, I'm still using Alf's model line trick.