I have noticed a bug where I go on a workout and when I press the back button or the home button, if I want to return to the workout all I get is a black screen. The workout is still going on in the background but there is no way I can get to it.
I had exactly the same problem with my 2 year old Gear S3; the screen would go black and could only be restored through switching it on while on the charger. Tricky when out and about ! So I took it into my nearest Samsung store (with support, not all do); they diagnosed a swollen battery, replaced it for £40 (since out of warranty) and it's now perfect just like new. So worth taking it in for. These batteries seem to swell if allowed to get hot just like some laptop ones do.