Can't assign "toggle pitch before duration" key command to certain Fn-keys

I have been trying to assign F19 as an alternate key command to toggle P.B.D, since it is directly above my keyboard’s numpad.

However, nothing happens. As a test, I then tried other Fn keys next to it:

F16 works, but not the three others. However, I tested assigning other things to F17-19, and those do work – so there’s not a conflict there. Any reason why PBD might not work on those? I’d like F19 because it’s ergonomically comfortable and easy for me to remember.

Is anything else assigned to F19? If so, what?

May be your operating system has claimed this key for something.

@k_b, if this is so, how could other functions work on it? (I don’t mean that rhetorically)

I previously had F19 assigned to open “Expression Maps” window which it was working with great success. I removed it though, before re-assigning, so nothing is assigned to it any longer. However, if I were to re-assign that to F19, it works. So it seems for some reason ‘toggle PBD’ does not like that key.

I’m on a mac, F19 does not appear to do anything in any other context (and I had no conflict otherwise before). Does anyone know of a mac utility perhaps where I can monitor any keyboard messages across the system, as a way to spy for any conflicts?