JdeBPIt always is. Fn is not a genuine on-the-wire key and what it does depends from the whim of the keyboard manufacturer.
Having to hold the #FnKey down all of the time defeats the nature of #NumericKeypads, I find. Part of the point, surely, is not to have to hold a modifier down whilst typing to do lots of numbers.
Although @Jayenkai can argue, at least, that pressing Fn means that one is pushing down on the keyboard on both sides of the knee and keeping things in balance. (-:
Speaking as someone who does not enjoy them, I diffidently suggest that you might not need it even now. (-:
I have a collection of add-on #NumericKeypads . One of these days I'll find someone flogging a cheap Canon DK1000i so that I can add it to the collection.