In the screenshot below from Calories In/Out > Activities, you can see that it's not adding up properly. The result is that the calorie display on the Garmin Connect mobile app is also wrong. This happens very often, probably 50% of the time, other days the total calories is correct (total calories = manual activities + automatic daily steps).
When this problem happens, the calories burned from Daily Steps usually also displays as "0", which is not correct.
This seems to be the exact same issue that is mentioned in this thread from over 9 months ago: https://forums.garmin.com/showthread.php?346825-Active-Calories-Don-t-Add-Up
Got a useless copy/paste response from Garmin Support, the guy didn't even read my email.