I don't have my computer here with ArchiCAD on it so I can't check this out but you might try adding a small perpendicular piece of the 12cm wall to the joint section. You will probably have to put the reference edge of the 25cm wall on the bottom side of your attached drawing and the reference for the 12cm wall on the top.
Gerald Hoffman
“The simplification of anything is always sensational” GKC
Archicad 4.55 - 29-Latest USA
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