Bangle.js 2 button stuck

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)

Ok, well that's interesting then - it almost sounds like a short.

What happens if you connect by Bluetooth and then do BTN.reset() - this will force the button's pin to be an output (it's inverted in software so actually should try and push out 3.3v) and then see what voltage you can measure?