fn not available for setting in "special key based visibility" / "Zusatztasten basierte Sichtbarkeit"

I want to hide some touchbar buttons, except I press the fn key, but the fn-selector is locked.
Is there any dependency or is this a bug ?

Fn requires the system preferences setting "Press FN Key to" being set to "expand control strip". Also it only works on current BTT versions and macOS > 10.15

Great - it works
thank you

I have identified a little problem.

To use fn as visibility trigger for some buttons i have to set "Press Fn Key to expand control strip".
So to send a function (F1 .. to F12) to my app I have to switch back to "Show F1 ... F12" in the settings.
F13 ... F18 are defined in the BetterTouchTool configuration, but F1 ... F12 are not available for selection.