Window mullion visibility in stacked wall

[redfish]

I have 4x4 windows in a stacked wall. The window mullion visibility is correct when placed in the top portion of the wall, but I lose the vertical when the window is placed in the lower portion. If I flip the facing to interior, the vertical reappears. Is this a family issue or a wall issue. I'm new to the family editor and stacked walls, so I'm not sure where I've goofed.