I have problems where Garmin shows calories incorrectly that were counted for an activity that starts before midnight and ends after midnight. The problem is it counts the calories for the activity on the pre-midnight date, but then it also counts lots of calories for all the activity that happened post-midnight during the exercise (it counts that part for the following day as general activity).
My FR630 seems to count the activity for the pre-midnight date, then also counts the total number of "steps" (activity it thinks you did due to exercise you started just before midnight), for the following days date. Effectively you have near double calories burned in Garmin. It will also sync this to MyFitnessPal showing up as a "Garmin Calorie Adjustment".
This seems to be a significant issue when doing any exercise sessions that happen to cross over the midnight timeline (i.e. you start an activity at 11:30pm and end the activity at 12:30am the next day technically). Does anyone notice this?