I'm experiencing the same issue. I have dark skin and am also having problems with my Samsung Ultra Watch showing erratic heart rate readings during workouts. My old Fitbit worked fine, but since switching to the Samsung Ultra Watch, my heart rate reading drops abruptly from 130 to 99 within milliseconds, showing unpredictable fluctuations.
My girlfriend, who has much lighter skin than me, tested the watch and doesn't experience any issues, so I know the watch itself isn't faulty. I'm planning to contact Samsung support tomorrow to see if this could be related to how the watch's sensors function on different skin tones.