The watch will need to get about 10 minutes worth of at least somewhat "steady" running (where the ratio of your speed and heart rate stays constant). With intervals, you are constantly either accelerating…
More specifically, your heart rate needs to be above 70% of your maximum HR for a sustained period.
What is your max heart rate?
The explanation is probably that it wasn't steady running just as Bitti says.
The watch will need to get about 10 minutes worth of at least somewhat "steady" running (where the ratio of your speed and heart rate stays constant). With intervals, you are constantly either accelerating (your speed increases but HR lags) or decelerating (speed decreases but HR stays up for some time). The watch's algorithms automatically discard those segments as "non-representative", so with interval training it needs longer periods of running.
My HR stayed well above the 70% level (about 112) for the duration of the 28 min workout.
What is your max heart rate?
The explanation is probably that it wasn't steady running just as Bitti says.
My theoretical max is 161 (I'm 68 years old) but I hit 167 in yesterday's workout and have had it as high as 175 recently.
You will want to have your actual maximum heart rate, or at least estimated, forinstance if you have hit 175 and it was with a chest strap or a smooth consistent 'good looking' wrist reading... and it was all out, but maybe you could have done more. (wasn't in a race, all out, blurry white vision..... maybe do 178 or 180.) With out a good maximum HR in garmin settings... it can't tell how hard you are really working!
Your short intervals wouldn't be best for a reading. Want good long (4-6min) intervals with fairly short recovery (1-2min) or best... a steady tempo run (warmup and then 10-30min medium hard (hour race pace)) to trigger a nice solid accurate reading. Do a short cool down (<10min), seems like when I have a long cooldown (unless its very cold outside driving HR down fast...) that it will see a slow pace and somewhat elevated HR (from workout) and think "oh he isn't that fast i guess... look how hard he is working at a jog..."
The estimated by age HR stuff is garbage - everyone is different - averages are just a blend of a potential range of probably > +/- 30bpm!!