I’m just sharing how the Watch and activity features work. From the second point in the linked article, “Every full minute of movement that equals or exceeds the intensity of a brisk walk counts toward your daily Exercise and Move goals. For wheelchair users, this is measured in brisk pushes. Any activity below this level counts only toward your daily Move goal.”
And from the bottom of the page, “Heart rate is one of many factors that Apple Watch uses to measure your activity and exercise. Depending on your workout, it selects the most appropriate inputs for that activity. For example, when you’re running indoors, it also uses the accelerometer.”
So when doing Yoga, the workout app probably dosen’t care much about your heart rate and rather other factors compared to a walk or a bicycle session.