It seems that H9 updates the image source based on a camera change, but it would be nice if the state of ‘Display Background Images’ could be (optionally) driven by whether you are actually looking through that camera.

Assuming you had ‘Display Background Images’ checked and this option also checked, if you are looking through the camera, the bg is displayed. If you are not (you move your view, switch to another camera (which does not have a view roto defined), whatever), the bg is not displayed.