I believe you also have elevate the grid system to be above the RCP cut plane.
It's all a bit of a work around really. If you have a multistory building it would be impractical with this method. Personally I use the layer system to draw my ceilings with view sets, pen sets and MVO settings on the floor plan window.
HTH.
Regards,
Nick
ArchiCAD 4.1–22, Mac OS 10.14.x iMac 27" 3,2Ghz Intel-Core i5