2-3 days a week my sleep data (Vivoactive 3) are displayed in Garmin Connect with start at exact 0:00.
sleep data before 24:00 is not available.
My vivoactive 3 does similar. I save the settings say to 2200 with a wake time of 0600. If I check in the morning to how much sleep I have had it can be anything from sleep start at 0200 to 0400 with a completely different wake up time. I think it may be that if I get up inside of night to go to loo it resets itself. Totally useless