Hey, so from your description, it sounds like your stylus emits a Microsoft standard signal (was built for Ms Surface behavior), once you hold the button down it emulates a "eraser tip" (like for styluses with an eraser on the other side of the stylus, when you flip them).
Krita, consider the Eraser mode of the stylus as another device (another stylus). So, try while holding it to select a brush in the brush preset docker 1/2