Hi ​@LBoeufSurLeToit, check if the HR was actually recorded - when you open the activity via the Workout app (the one with activity rings icon) on your iPhone, and then tap the details (“view more” or similar - sorry my phone isn't set to English so I’m not sure about the exact wording), you will see a graph for all metrics, including heart rate. 
 

If it was recorded properly all the time but not synced to Strava correctly, you might need to submit a support ticket. 
 

But if you see the gap / missing HR also in the iPhone Workout app, then the HR sensor likely didn't work correctly for a while - sometimes it's because the wristband was too loose, or because of the cold environment (the optical sensor reads HR from the thin vessels right under your skin; but if it's very cold and the skin is exposed, the vessels shrink and no blood flows to them, and as a result, the HR sensor can't read anything). If that's the case, Strava simply didn't get the data. 

Hi , thanks , culprit was a customn workout on Watch app with a warm up period , duh...

Do you know if the work done in a  warm up on the Watch  is sent to Strava to count in RE ? Or is it excluded?