Hi Andrew,
What version of Survey123 are you now using, is it the latest 3.5 release, can you give exact build number from About screen.
What exact version were you using before when it was working?
If you install previous version again, does the stylus work normally? Is the stylus working in other apps?
There should be changes to the app that stop the stylus working, as it treats mouse clicks, finger taps and stylus all the same.
Phil.