First run with Watch 8 was disappointing, as I had read reports that heart rate monitoring has been improved with this watch. I didn't experience just minor issues--the Samsung Health app showed my heart rating dropping very low when going uphill and getting very high while going downhill (i.e., opposite of reality).
Would be helpful to hear other people's experiences...
I am having issues while biking or swimming, and maybe while walking. My issue is different than yours. I have a Watch 7. While in bike mode or swim mode the heart rate is absurdly high. I used an oximeter on my other hand, which reported heart rates 40 to 80 bpm less than what the watch was reported. The issue continues through the watches cool down monitoring mode... then snaps down to the correct heart rate.
