at the gate

The plane shouldn't start moving when the game starts, even if the engines are running...

Whatever the default flight's plane is doing when you load a custom flight, your plane will be doing. So If the default flight is a cold and dark plane, every custom flight will start out cold and dark. If the default flight is with the engines running at high speed, then when you load up your engines will be running.

Try saving a new default flight.

Be sure you R nor in autopilot mode and and your speed not engaged

You are not alone, this happends to me to... dont know how to fix it though 😞

Traches already pointed out how to fix this. Here it is in more detail:

In Free Flight select the Cessna 172 SP

Location select KSEA

Time and season select Spring, Day

Fly now

Once the flight has loaded, press Ctrl+. (this will put on the brakes).
Make sure your throttle is at idle.
Next, press ;
The dialogue box to save a flight will pop up. Give it a name such as "New default flight". Now the most important part: Check the little box in front of Make this the default flight.
Exit the sim. Load the sim again, now you have a new default flight. If you select Fly Now you will see that the brakes are on.