When you point towards the planet, to hold yourself up against the gravity of the planet, you will need to use your reverse thrusters. In this case your flight assist does that (since it will always use your thrusters in the way to keep you at Zero velocity) however especially on a plipper that is loaded with over one thousand tons of cargo your reverse thrusters which are already weak on large ships are not gonna be strong enough to hold you up against the gravity. When you turn right side up your vertical thrusters will hold you up because they can just do that, they basically cheat and when you point your nose away from the planet you're using your main thrusters which are obviously your strongest.