Hi! So, I've had the same issue in 2.78 with the sculpt tools. I've ended up having to keep the ortho view on the X/Z axis (numpad 1) for the cut to work. I changed symmetrize to "-Y to +Y" ("+Y to -Y" works too), and it works for me. It's just not working when the axis is on Y/Z (pressing 3 on numpad). I wonder what's wrong there!