Same problem here in France with my Galaxy watch. I'd like to add it was exactly the same with my previous (and stolen) Galaxy Sport and the same with my Gear S3 .. so sounds like a known and steady issue.
Works perfect in normal life but as soon as you want to use HR tracking during sport activities , no luck.
I've yet to try to wear it in reverse but I pretty much gave up on this and using a different brand for real sport tracking.
DO we know if Samsung is aware and working on this ?
Even after so many software updates...this problem has yet not been solved. My newly bought galaxy watch as well stops heart tracking ocassionally. Samsung needs to level up!!