Clipper attribute merge confusion

Hello,

I am stuck on a problem that is probably very simple, I just don't get it...

• I have a simple test IFC file with two rooms and some random furniture:

• I would like to use FME to run an inclusion test, which furniture is in which room.

My thinking is to extract a center point of every furniture instance and run a spatial join using the Clipper tool. Something like this:

Now, the confusing part.

• The clipper attribute merge is telling me all the furniture are in just one room:

Why is that?

It works fine using an AreaOverlay, but I really need a 3D inclusion test...

Test IFC and my workspace:

Thanks.