On any flight, regardless if it's the first flight of the day or not, they connect the jetway way before actual boarding of passengers.
You are still missing the point. If it's not the first flight of the day, you are supposed to start with Deboarding.
Plus- nobody starts a new flight "right after" they just landed without restarting the simulation.. .
This is irrelevant, since GSX doesn't really care if you restarted the sim or not.
It's you that, if you know you have landed from somewhere before, even if you restarted the sim, should start with Deboarding first.
Again, this is what you are supposed to do, except if you are simulating the first flight of the day. If you are not, how do you expect passenger and luggage from the previous flight (yes, yes, you restarted the sim, but it doesn't make any difference) to exit the plane ?