I'm still having this issue and I posted in another forum as well. Link to the thread is here.
I also opened a support ticket. I don't get why the active calories show up sometimes as less than your timed activities. I checked and it did on more than just the one day I mentioned in the thread above. It does it on many days since I got my new watch. I will do 500 or 600 or 900 calories (or whatever it is in timed activities) but then it may show only 270 or 325 "active" calories on Garmin Connect?? But on the phone, website, and device if you look at the activities it still shows the correct total of all the calories. It's just the "active" calorie that is off. How can active calories be less than the timed activities? Active calories should be a combination of all your basic activity (basic steps that your tracker logs and calculates some calorie expenditure for) plus all timed activities/workouts. It should at least be that much.
How is it coming up less for active calories? Again, shouldn't it be at least the total of all the calories from all the timed workouts that day? This is weird. And seems like a bug. Any thoughts?