Same for me. If I wake up, it still shows it as core sleep.
Further, my sleep tracking ends with sleep profile - e.g. if my profile ends at 5:30 AM and I wake up at 8:30 AM then it shows I slept only till 5:30 AM.
Worse part is, the data between 5:30 AM and 8:30 AM is stored with Apple, but visible only with 3rd party apps.