Restart client means closing the NoMachine client window on the Windows PC, and then starting the NoMachine client and connecting to the Mac Server again. It doesn’t require a restart of the PC itself.
The fn key is stuck on the server side, but restarting the client (as above) clears the problem on the server, at least for a little bit.
I can also clear the fn key on the Mac server by pressing the fn key on the Mac Laptop keyboard itself, but it immediately becomes stuck again as soon as I press any modifier key on the client (Shift, Ctrl, ALT, OS, CapsLock).
I’ll post again if I can find some repeatable way of getting the fn key stuck. So far it seems sort of random, but it usually happens within just a few minutes of connecting.