Was the original covering lost from the top AND the bottom of the wing?
If only one side, then it could fly as described.
If both sides were lost, then the chord shortened a WHOLE lot, and the c.g. ended back to the trailing edge of the "improved" wing.
Which would have resulted in a plane-shaped hole (front view) in the ground.
With only one side lost, there's no path for the lower side pressure to the upper side thru the extant covering, so the plane would continue to fly.
As the GL has a center-of-the-rib spar set, the initial description seems odd..
Unless the only covering lost was that aft of the somewhat short-spanned leading edge sheeting on the upper surface of the GL.