Bit of a Necro-post, but I was having the same issue including focal length, which whenever I would edit it it would just snap back to a default of 35mm, but I was able to solve it.
the Cine Camera Rig Rail actually inherits the Attachment’s (i.e Camera’s) settings by default. It’s my understanding that the individual points on the rail can actually control (e.g. keyframe) the Camera settings so that any Camera attached to the rail with automatically adopt the settings, but also the settings can be animated on the rail points. I’m not really sure how to do that though as I haven’t had the time to explore that further.
Anyway, to make sure your rail doesn’t inherit the camera settings so you can just adjust them on the camera itself like we usually do, simply select your CineCameraRigRail and in the Details panel go into the Attachment section and uncheck the parameters you want to adjust on the camera itself
In the above image, I’ve unchecked all the Camera settings and I was able to modify focal length, aperture, focus distance manually on the CineCam as usual.