This just happened to me over the weekend on my Galaxy Watch 42mm. I've had it since Christmas 2018. It happened during a hike, when I noticed that HR wasn't tracking. Went to clean off the sensor, and the glass moved with my finger! Managed to keep the glass on, but now it rotates around in place. Between this issue, and workouts that randomly stop tracking, I'm seriously considering a different brand for my next watch.
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