isn't drag at certain point be equal to thrust ? If drag is based on the speed, and because there is no speed increase deriving from thrust (my immaginary airplane has no starting speed) we will get that thrust = 0 so what's left is only the negative force of drag that will increase as my plane falling because it will increase the speed too right ?If drag is backward, that means that initial velocity was forward. Accordingly, the plane will move forward. It will not move as far forward as it would have done without drag, but it will move forward.
If your step size is large enough that the plane stops and then moves backward due to drag, then either your drag formula is incorrect or your step size is too large. Drag scales with velocity. The slower you go, the less the drag. You will never stop and then start moving backward under drag alone.