Heart rate is recorded only intermittently during workouts

I have heart rate tracking turned on, I have low power during workouts turned off, and my resting heart rate is tracked during the day and while I'm sleeping. I wear the watch tight enough that it is hard to move it around on my wrist.

As an example, I went for an 18-minute run yesterday and only the last 3:47 of heart rate was recorded, and even that had a break of about 30 seconds in the middle. Recordings seem to have been taken every 10 seconds for the 3 minutes after I finished. It was about 48 degrees outside so not particularly cold. This watch is marketed as a workout tool so there must be some way to get this to work. What am I missing?