Nothing is wrong.

For a plane to fly, it must create lift. The angle of attack of the main wing and the shape of the main wing work in concert with the air speed to create lift. The angle of attack is adjusted in flight by the elevator. The elevator must counter any additional nose weight the plane may have, depending on how its balanced in order to maintain this correct angle of attack.

For a plane that is trimmed for normal flight, you create a stable situation where all the forces of lift exactly counter the forces of gravity, Depending on the plane, this is normally a level attitude.

Now invert the plane. You have completely changed the forces of lift. The angle of attack must be adjusted back to the static condition. This normally requires some down elevator to bring the nose back up. You could trim the plane in this condition if you so desired, but when you go back to normal flight with all that down elevator - guess what happens!

A symmetrical air foil will reduce this tendency, as the angle of attack adjustment is normally very small. For a flat bottom wing, the adjustment will be much bigger since the difference in lift is so much greater.

For a neutrally balanced plane, the adjustment will also be smaller as there is less elevator adjusting for the nose weight. Thats why aerobatic planes have symmetrical airfoils and are neutrally balanced.

Brad