The shortcut never reached the recorder
Another program can own the same key combination: a game, Discord, an overlay, a second recorder, or a keyboard utility. Windows gives a shortcut to whichever program registered it first, so the recorder never hears the press. Try the shortcut on the desktop, then change it to a combination nothing else uses.
In Flashback: When you set the save shortcut, Flashback checks with Windows before accepting it, so a reserved or already-used combination can't replace a working one.
