I'm convinced to send it back in service
I wouldn't keep my hopes high. From the gigantic amount of users complaining about this I haven't read about anyone who had this issue and he got the watch repaired or replaced conclusively resolving the problem including myself.
Bought a Galaxy Watch Active for my wife and tried in gym today both watches (left hand with my Galaxy Watch and right hand with the new Galaxy Watch Active)
See the running on my Galaxy watch with flatline for almost 15 minutes (from 30 total workout) and below see the recordings from Galaxy Watch Active.
My watch has been collected today for the 5th time by Samsung for repair. The motherboard and HR sensor has been replaced twice. It will be repaired for the 3rd time (I am assured) before they will consider any other solution to resolving the none functioning HR monitoring during workouts. Don't know what they did the other two times they had the device. This watch is just like their Note 7 and Galaxy Fold smartphones, it should not have been released. I would like to see BBC Watchdog follow up on this issue and the poor after sales service of Samsung.
Try shaving hair off wrist and tighten watch strap so watch can not move around, worked perfect for me today on 1.5 hour cycle, first time I have not flatline since owning the wat h