You misuse the term 'active' - only one section-plane can be 'active' in any given context.
Having the nested section-plane allows you to cut what's inside its context [when it is active].
Another section-plane outside of that group's context will cut everything in its own context, including the group which has the nested section-plane.
You need to have both section-planes active to hide the loose geometry that is not inside of the group.
Here's some screenshot showing it - in the original there two active section cuts and the bigger cut is looking towards the loose geometry, so it is visible - because it's outside of the group it's bound to be seen.
The second shot shows the section cut in the opposite direction this time the loose stuff is not seen.
Trying to do this with section-cuts alone is going to be very convoluted...
If you want to hide that loose stuff I suggest you use the section-cuts as you have made them, but put that loose stuff on its own layer - which you then switch off in that scene - see the third screen-shot...
[image: jjfP_Capture1.PNG]
[image: 3u1q_Capture2.PNG]
[image: qQcQ_Capture3.PNG]
