i have started facing this same issue on my vivoactive 3 music.. first i discovered that despite turning off Bluetooth and wifi they are always on... confirmation received from Garmin support twice...bummer!!!
and now from past week or so every data sync makes changes to my sleep data... eventually ends up wiping out deep sleep data ..and merging deep sleep duration with REM??? god knows what kind of algorithm Garmin (which havei consider to be one of the most reputed brand) has come up with
going by this pace, it may just be a matter of time when I discover that heart rate and steps counter are gimmicky too...
I was bushed last night. I fell asleep before 8:00 PM in front of the TV. Got up and transferred to my bed around 9:45 PM. Went to the bathroom around 1:30 AM and woke up just after 5:00 AM. My VA3 said I slept for 48 minutes. I understand that it might have been difficult to figure out my start and stop times for sleep as I had gotten up a few times after my start.
I adjusted my start and stop times and now Connect says I slept 3 hours and 28 minutes when it was easily 8 hours even allowing for what I believe is my real awake time.
My problem is that when I look at the movement graph, there are large swaths of time where the graph shows almost no movement yet it thinks I was awake. Additionally, there are small sections of the graph where movement is slightly higher than some of the awake sections yet Connect believes I was asleep.
I have noticed that sleep times are adjusted slightly if I look at the graph later in the day but it will never show anything close to reality.