Did you try turning on the double sided property in the mesh properties? I can not guarantee it will work but it usually helps fill up the holes on my meshes.
Possibly add more edges around the corners where the object bends, I had a similar issue when creating a object with inset curves where the solution was to add more edges to support it. Additionally, the topology on the edge could be better.
Turning on “double sided” does not solve the problem with your mesh, it just disguises it.
The backwards faces are still going to react to light the wrong way.