Lightmaps are still very limited for props, and only really useful if you design the prop specifically for them.
What's happening here is that both sides of the metal sheet share the same UV map space - or share the same texture, basically. So for that area of texture, the lightmaps are being calculated twice, once for both sides, and then added together. Because the other side is in complete shadow, it makes the front face completely dark as well.
It's a horrific limitation that no other engine has to deal with, but unfortunately there's no easy way around it without modifying the prop and it's material entirely.