Graph and data indicate I was awake the hole night though I wasn't

This has happened two nights in a row. I use the app with GalaxyWatch 5.

Hello, usually, when the false-positive awake covers the whole duration of tracking, it is triggered by Awake when using phone.
At the moment this feature relies on screen-on times to detect your awake state, detecting the shaking of your hands and orientation of you device. So it marks as awake only if phone is at least ~10 degrees from horizontal position and there is a minimal signal on the accelerometer which shows the phone is probably being held in hands.
For example, when you have the phone charged in a stand, charging causes screen on and the sensor is very sensitive for instance. Can you think of why in your case this is leading to awake signals?

You can lower the sensitivity in Settings > Sleep tracking > Awake detection.

You can also revert the awakes estimated by the app - either all of them at once, or individually: