Hey,
It's that setting that is bugging out, at least for me. Regularly I need go into Options and toggle it off and back on so cursor stays caged.
There isn't any clear trigger for bug as I have gotten it after random conversation, after random battle, during exploring etc. It just stops working and cursor goes wandering to 2nd monitor. Then I need go toggle setting off and back on to prevent cursor from wandering to 2nd monitor.
Edit: Running game through Steam on W8.1