Apple Watch not recording bike for exercise ring

My Apple Watch version 6.2.6 will tell me the distance, time, heart rate, etc., for an outdoor bike ride, even congratulate me for “cycling workout record,” but then not close my exercise ring. Yesterday, I rode over an hour, 8.38 miles, and it only registered 13 minutes and didn’t close my ring. What gives? Is there a minimum heart rate? (Although mine was well over 100). My daughters have had the same experience, always with the cycling setting. So far, the Apple standard response is about a standard “brisk walk rate”, etc., but doesn’t explain what that is.