This happens when the tracker is unable to get a correct GPS position, in a garage for example.
Do not worry. Your vehicle is still protected in case of theft: Trackting will alert you immediately and provide an updated location as soon as it locks on to the GPS satellite signal again.
The reason it works this way in “Parking” mode is to avoid all unnecessary battery consumption. Therefore, if the device fails to get the correct GPS position in just a few seconds, it goes back to standby mode.
In “Alarm” mode, the goal is very different: we want to read valid positions as soon as possible. This is why the tracker stays on when your vehicle moves, up until it gets a valid GPS position.