@Jose have found the fix is simple. Switch Bluetooth off on the phone and re enable it when you wake up. I think the app has a bug in that if you do get up in the middle of the night and it syncs, even though the watch will change the sleep time if you go back to sleep - when you wake up properly the app won’t accept the updated sleep time.
Since I’ve done that sleep tracking has been excellent. Hopefully an app update is soon