Note – I am only seeing the AO through the “backside” of the material – IE it’s a thin surface with no thickness, from the frontside, the material renders correctly, through the backside (with double-sided faces enabled) you can see the AO through the material.
I have been able to recreate this issue. Its very strange.
B3D 4.0.1 V3D 4.5 OSX
Its rather strange in that it acts differently if the surrounding walls have forward or reverse normals. When the normals are flipped facing away from the camera when AO is applied its as if the face with a double side material takes on a transparent look from the back side.