I see what you mean.
I think you found the one case that is not implemented. I think ARCHICAD simply does not calculate cut geometry for Beams when they are horizontal.
It only does it for inclined Beams but I checked and it seems that the incline angle has to be at least 0.0001 degrees or something for it to generate the horizontal section, so it won't work with zero degree inclined Beams either.
This is a limitation.
So, based on this, I think the wish should be called: "True Horizontal Cut for Horizontal Beams" or something similar.
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