Hello Tom,
I opened your RWA_Sliding Sash 1 in the lib part editing environment
and commented out the model wire command and the repeat
macro calls with the model surface command.
I un-fixed it to try to reproduce the missing lines.
After doing this I could not get the joint line between the upper sash
and the window frame to disappear.
This object has some strange behavior.
In the lib part editor the 3D view shows the material for the
the frame and sash the same (as I had set them in the parameters)
In the 3D preview they show as the same.
In the 3D window the frame is the material I set but the sash material
is what appears to be "Wood Pine" even though the sash material
is set to " Paint-Whitewash".
In the 3D window and in the section/elevation window
the lines that are missing in your .jpg are present.
Also, the window you show in your .jpg image
is different than the window object RWA_Sliding Sash 1.
(see attached image)
The window in your .jpg image has coped tails on the
upper sash side frame members.
Anyway, I can't study the missing line issue because
I can't reproduce it on my machine.
Peter Devlin