If you parent a camera to a null object and then animate the null the motion blur effect won't be calculated in the final render. The same thing will happed if you animate camera on a path/curve. I hope it's a bug because otherwise it's a big issue where there is a very limited functionality to camera use if you need motion blur to happen in camera.
Haven't tested in earlier version so don't know if it worked fine there. Doen't work as expected in 27.16 OctaneBlender build.