If I understand you correctly, you are not seeing a surface pattern on the top of a wall that is located below the cut plane. Basically, the problem is that Revit does not give us an option to wrap material across the top of a partial height wall. If you hide your roofs in your 3d view, the tops of those walls will still show as blank. I played around with "split region" to see if I could just chop the top 1/2" of each layer and turn that into gypboard, but no luck.
In order to accomplish what you want to do, you have two options. The first way will be more efficient, but may not quite work and I'll explain why. In your plan view, hit the Visitability / Graphics Overrides, and scroll down to walls. Under "Projection/Surface:Patterns", click the override and make it the light gray solid fill. The issue with this is, if you have some existing partial height walls, they would be displayed in the same gray tone. The phase filter overrides are trumped by the view's visitability/graphics overrides.
Option two takes more work, and is not "smart." With the Paint tool, go into your plan and paint the tops of the partial height walls with the same material as your walls finish material. You will have to make sure to update though as your design gets modified. Using a section box in the 3d view will help to be sure you don't miss any.
As I finish this up, I realize there is another option. You could create a sweep with a profile that will match the thickness of your finish material and your wall's overall thickness. You could position that in a way that it will be on top of the wall, and it will display it's surface pattern in plan. However, you will see the line in elevation so it would take some work to flesh out the method...
