I have the same problem. My temporary work around is to use the Edit:Display Order function to push one of the slabs / fills backward and try again. Works.
Hint: show the Display Order pallet on the toolbar. Its always in reach.
Intel i7-6700@3.4GHz 16g
GeForce GTX 745 4g HP Pavilion 25xw
Windows 10 Archicad 26 USA Full