Archimax,
You've run into the 'emperor has no clothes' of composites in ArchiCAD.
Composites are not real building elements, but an automatic 2D fake. The wall is treated as a single mass and the composite multi-line-multi-fill is simply displayed in plan and in section as a sort of automatic 'patch'.
So, if you cut into a composite, you do not see the skins remaining in section/elev or 3D, instead, as you have seen, the material and associated fill of the reference line side (or opposite) is what always shows.
To have a different surface fill/material show, you can either do as farzinassadi suggests and place a thin wall at the back of the niche ... or you can create the niche with a solid element operation rather than using the niche objects. If you create a cutting mass to subtract from the wall and create the niche, you can assign a material to that mass, and when you do the subtraction, choose the option that says that material/fill of the cutting mass should be the material on the resulting surface.
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB
