I ran a 8500gt card on my old vista system with v12 and it worked okay. Now that I have upgraded to V13 and a Win 7 64bit system with dual 9800gt's I am having similar issues as you plus the redraw is slow. (3D works fine though) At first I thought it was the video cards but the 8800gt is an archicad recommended card and the 9800gt's are virtually the same. So maybe you are right and it is an archicad bug??
V12-V27, PC: Ryzen 9 3950X, 64g RAM, RTX5000, Win 11