For me Win10 AU gave me back the top button. I'm using the SP4 pen on my SP3 and I've not been able to get the Surface app to have the options for setting the function on the top button (had it once briefly with SP3 pen drivers but then it disappeared with SP4 pen drivers) despite repeated deleting and reinstalling pen drivers and Surface app.
Win10 AU has settings for the pen buttons in the Settings and I can now set single click, double click and click and hold to launch and Windows Ink app or any desktop or MUI app.
Have you tried removing and then re-pairing the pen in Bluetooth settings?
If that doesn't work I'd try unscrewing the pen top and removing the batteries and then returning them (careful to keep the + / - alignment correct) and then re-pairing. And if not sure the batteries are still good, try replacing them.