Heart rate iwatch 8

I have an iwatch 8 GPS, when I go on outdoor runs and record the run with my watch using the built in native outdoor run workout for some reason the first 1/2 to 1 mile the heart rate is always 160, once the watch finds the heart rate it records it for the rest of the run. Tonight was 6 miles and I am missing first 1/2 mile. I run without phone and sync up when I get back.