Thank you Will!
This worked for me in My Fenix 5 today :)
Just did it on my 935 today - solution still relevant
I have the same issue. Did the hard reset and the problem is still there. Heart rate is picking up correctly but not logging. Please help.
Hopefully you've resolved now, but in case it's of use to others I also fixed mine with a simple switch off and on. Using Connect I could see that it had stopped logging after a I ran the battery flat mid run a week ago. First time I've lost logging, and first time I've run battery flat mid activity, so maybe a connection.
Perfect, soft reset worked great for just this fault