How come the Sub Group button on a fader only works when the fader is 0%, and doesn't work if the fader is up?

When the fader is above 0% this button Assert function.

If you have the same channel in 2 LTP subs one at 50 % and the other at 75%, pressing this button will Assert the value in that sub.

If you always want Group Sub to can change the button in Fader Config, Tab 36.