Why does Apple Watch reduce the calories data recorded by another fitness tracker in Health?

How far back did the previous data go? I never look in Health for workout data, I always use the Activity/Fitness app to track workouts. If you are referring to such items as past 7 days, past 30 days, past 12 months those are averages and will be affected by newer workouts. That is not a bug. But if you think it is click on the Get Support link at the top right of this page.

In some studies that have been done the Apple Watch is most consistent at counting calories and exercise. If you wear 6 fitness trackers while working out you will get 6 different results, stick to one. And no, it isn't a bug that has been around since 2015. Stop comparing to your old tracker.