I’m unsure if I’m right about this but it seems to be true for me. This relates only to 3.1 on a PC.
I was having far to many issues with ZSpheres and got to where I did not use them. I had another program I had to reduce hardware acceleration on an NVidia 8800 GT to solve issues. I forgot to change that when I went to ZBrush. I decided to give the ZSpheres a go and for once had zero issues other than the incredible shrinking trick. It would be worth trying for others if you just can’t get those ZSpheres to cooperate. I took it down two clicks in the troubleshooting tab.