You can't change the FOV in Normalized Z axis-mode, because it's mathematically impossible.
Normalized mode means that for a 100% zoom, the camera is at Z=100 (the "default camera Z"). To achieve this with any viewport size, it adjusts the FOV to meet these requirements. If you changed the FOV then Z=100 would no longer correspond to 100% zoom, and so you're not using normalized mode any more.
This is why you need to set the Z axis scale to Regular in order to change the FOV - which in turn changes the default camera Z (which you can see in the 3D Camera object's properties).
Normalized mode is designed for 2D games, so my advice is if you are using any kind of 3D view with the 3D Camera object, then you should switch to regular mode.