Have tried using the Sleep as Android app two nights in a row now. Each night I've begun the tracking on my watch and it is active. I can check the phone and see it is tracking.
I then put my phone down (screen down) and double press my watch button to shut the screen off. In the morning I activate my screen and the notification that sleep tracking is active still shows; I pull it up and "Stop" tracking.
Then checking the phone it says I slept for 3 minutes. So something has stopped it on the phone almost immediately but the watch thinks it is still going.
Phone isn't dead, has been plugged in all night, battery 100%. Wifi is on, is connected to the watch.
Phone is OnePlus 3 running Android 7 and Oxygen OS 4.0.3