Bangle.js 2 button stuck

the pads on the PCB look a big dull (although that could just be lighting)

from another angle/light it is actually clean. I'll try to clean it anyway.

How old is the watch?

1 year

being pulled up to 3.3v via an internal pullup

this is the strange thing, reading the value between GND and the button pin with a tester I detect a voltage of only 0.09V (no it's not a typo)

There is a vanishingly small chance that the unpopulated component you found could be an optional pullup resistor

I look at this here: https://www.espruino.com/Bangle.js2+Tech­nical, it seems that the component is also missing.
and in any case given the proximity it should be a component of the compass.

you can effectively disable the button

yes, I've already tried, it works but I'm trying to understand if I can get it to work again without using the "tap" trick