I've done some experimenting with my HRM Pro and Fenix 5 plus and i think there is a bug.
If i start an activity wearing the strap and then move out of range of my watch (leave it in my car for example whilst i go do a cardio workout) then come back to the watch mark the cardio session as finished. the strap and watch sync okay. the steps and intensity minutes are recorded correctly from the strap and added to garmin connect and the watch.
However if i start the activity waring the strap and leave the watch in my bag but within range of the strap then mark the cardio session as finished the steps and intensity minutes aren't recorded from the strap. its as though it thinks I've been wearing the watch and ignores the steps and intensity minutes from the HRM. It does capture the heart rate data from the strap though.
It seems that if the strap is within range of the watch during an activity it doesn't record steps or intensity minutes.
This appears to be a bug that Garmin should look to fix.
Does anyone else have this issue?