I might be wrong because I do not currently have Grasshopper installed so I cannot check it, but I think that Openings have not yet been implemented in the Archicad-Grasshopper Connection. (There is no word about it in the User Guide).
So I think you cannot expect to get correct results by the Decompose Nodes when working with Openings.
Try to place those openings into the Slab by editing the Slab and creating Slab holes.
In the case of openings in walls, place them by placing an empty "Door/Window Opening" library part the wall.
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