In UE 4.26.2, it seems like applying materials has no effect on the BSPs used to build out our test room. I’ve created the “Wall” material instance, and for each wall box BSP I’ve added to the level, verified that the “Wall” material instance is assigned to the material slots on their faces. However, the viewport only displays the default “no material” texture.
I tried to do this as well, and the materials still wouldn’t render on this geometry. However, when it was converted into a static mesh, the materials applied just fine.