This only happens if you draw the wall independently. If you select the 2 walls and right click and join, this will not appear like this. You may need to go back and remake the wall as existing. > right click >edit wall> existingviewtopic.php?f=12&t=78&start=20
last post touches on this.
We could make it so an existing wall can be changed by the end user to suit their needs either hatch on top or colour. I think if we dictate a colour or hatch by default many drafts people may not like the style , colour or texture in the 2d Layout file.