Continuous Heart Rate Readings with Apple Watch

I recently tried using my Apple Watch Series 7 to do running workouts. I want continuous heart rate readings for deeper analysis in tools like GoldenCheetah.org and intervals.icu. I use the Apple Workout app on my Apple Watch to record my running workouts. I use the HearthFit app on my iPhone to extract .FIT workout files to my iCloud account. I then transfer my workout data to the above mentioned analysis tools.

To my surprise, heart rate is sampled no more than every 5 seconds. Occasionally, there are gaps of 15 seconds and more. In contrast, I can connect the same watch to a Polar Verity Sense optical HRM via Bluetooth. Heart rate readings are then made about twice a second, with no gaps or glitches.

What is going on here? Why is HR monitoring continuous only with the external Polar HRM? Is my Apple Watch too old? Is it defective? Or is the Apple Watch incapable of sampling heart rate faster than every 5 seconds?

FWIW: Workout > Low Power mode is off, HR access is granted to apps on my watch and phone,