Hi,
The reason why the whole wall gets overridden is unfortunately how GO works - it overrides whole element.
There is a workaround though - BM (Building Materials) are using specific pens to display hatching. You could create a pen set where both fill pen and fill background pen for wall's main BM are "brown" (like in your example) and that is how you would get Solid fill for a wall leaving cavity transparent.
You can find pen numbers in BM settings (I recommend not overriding them in there but instead create special pen set for the view you want)
Łukasz Pietraszko
ArchiCAD 26 PL, since AC20 | WIN 10 Home
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