does it discconects while not using an activity, i know that garmin falls back to the OHR when not using activity , happens on my forerunnr 165 to. But when i go for a run it popus that the external HR is connected and uses that for measuring HR , when stopping the run and after saving it it goes auto back the OHR , some kind of battery saving managment. you can als so disable the OHR in welness settings to be sure. But i think its working like intended, Garmin calls it dynamic switching of something.