Plan Region not displaying quite right

[iandidesign]

I have some high windows and on my floor plan I want them to look like my other windows (i.e. cut) even though they are above the cut plane. That’s what a Plan Region is for, right? Well it works fine except the display is slightly off. The glass does not show. And yes I checked, and rechecked, that the region cut pane passes through the glass, not the top or bottom rails of the sash.

Have a look at the screenshot. Both windows are from the same family. Any ideas?