It may be the same problem that you can see in OpenGL mode, in that the glass displays different levels of transparency on different panes of glass. I assume the effect must carry over into the sketch render too.
I did moan about it in Oct 2007 along with the now-fixed doors, but they are still the same - see attached image.
http://archicad-talk.graphisoft.com/viewtopic.php?p=103289&highlight=#103289
I wonder if it has to do with the order that the panes of glass are modelled in the GDL script, or whether they are mirrored (mulx -1) etc?
Sorry I can't help further.