Hi Konrad,
I've just tried this myself. I turned Write on, on a track, and then I started playback. During playback, I tweaked the volume slider, and I could see the automation being written to the track. I then turned write off, and played the project from the beginning, and the volume fader moved up and down in response to the automation.
So I don't know what has happened at your end Konrad. Have you accidentally turned something on or off?