I think there’s a bug with BSP object’s Details tab. In the editor sometimes the Surface Properties will disappear randomly.
edit: apparently it’s not random. Clicking on a brush object in World Outliner tab will hide the Surface Materials and Surface Properties. While clicking on a brush object in 3D view window will show them normally. Also I noticed that the material list of a brush sometimes shows different material(s) although visually the brush is still shown with correct material. Clicking on the brush few times will bring the correct material list in Surface Materials tab.