I'm experiencing something similar to @Bigbird running v42.1 on a mac m1 mini Monterey 12.5. For some reason when F.lux begins on startup, I do have the top menu icon, but when I try to choose about or preferences, nothing happens. Then if I quit F.lux and restart it, all of those items work fine. I'm reasonably sure I don't have two instances of the app or something.
This isn't a big deal, just a minor annoyance. I seldom need to go in to about or prefs. Just reporting it as an FYI.