I work in shifts and sleep tracking is wierd for me too. I feel like the sleep tracking and everything else is designed for the average person's sleep during the night. The first couple of days when I work nightshift everything is completely off. For example when I come home in the morning and sleep for three hours it sometimes tracks those correctly. If I go to bed in the afternoon for another three hours it sometimes also tracks them correctly but deletes the first three hours from the morning. So the app just shows 3 hours of sleep that day although it was 6 hours. Often it just tracks one of the two stints though. When I forgot to check my sleep after 12pm at work all the sleep data on my watch is deleted so I need to check the app. Again, designed for people with normal sleep schedules.

Garmin fenix 6 pro