Unwritten Rule: Walls that are unconnected AND less than 6' in height will not display their interior layers (you'll get a display similar to coarse).
These two walls are the same family, just different heights...
Constrain the Top of the wall to the same level as the base constraint, but give the wall a Top Offset equal to the desired unconnected height. This is essentially the same thing as being unconnected, but in Revit's eyes is not...
Same as above, but constrain the top of the wall to the level above. Do some math to include an offset "down" from that level.
"Attach" the wall's top to something... In this example, a Reference Plane will do.