I briefly looked at the source for this and I think the issue is the use of XNVCTRLQueryTargetAttribute which seems to be broken.
The nvidia-settings application uses NvCtrlGetAttribute directly (ie. it doesn't use the X event mechanism) and it works fine. Not sure what the problem is with XNVCTRLQueryTargetAttribute.
I briefly looked at the source for this and I think the issue is the use of XNVCTRLQueryTar getAttribute which seems to be broken.
The nvidia-settings application uses NvCtrlGetAttribute directly (ie. it doesn't use the X event mechanism) and it works fine. Not sure what the problem is with XNVCTRLQueryTar getAttribute.