A guess resulted in a neat workaround...
On the chance that Music might have become corrupted, I called on Pacifist, but it was unable to reinstall it (because Apps are read only in Catalina), so I extracted it to my desktop, and that incarnation responds to the F keys!
I'll live with the issue until 10.15.3, and if that doesn't correct it...reinstall time. (Hmmm... joemike recently posted that apps are writeable in 10.15.3...we'll see what happens.)
