Request has been made previously - referenced (
https://answers.presonus.com/56193/can-the-faders-for-universal-control-sub-mix-faders-activated). Question never answered on that page.
The new release, Universal Control 3.4.0.61835 Win x64 (Nov 13 2020), still does not address this bug. "Mix 5/6" fader still has no effect on headphone level.