This default also seemed odd to me, but after some search found the solution.
I have a Korg nanoKontrol Studio with 8 faders setup as MCU/Mackie.
I could page right/left on the tracks, but the hidden channels remained active on the Korg regardless of Studio Ones mixer layout.
To fix this, in the left side of the Mixer, in the channel list at the bottom is a "Remote" button.
If you click this, the midi controller faders will remain in sync with the displayed channels, removing the hidden ones. The best way to see this is with the Presonus iOS Remote app.
Another feature is, if you then toggle of the remote button off, that fader setup remains in place on the midi controller, and you can change the GUI in Studio One's mixer without effecting the midi controller. Toggling it back on will bring it back in sync with the GUI.
One shame is that the Main outputs / Listening bus etc aren't included in this mapping, so can't use the right hand faders on the midi controller for them.
Hope this helps