UV unwrap ignores mirror on part of the mesh? So I made this character, or half of it while mirrored along X, as you can see here  

But when I apply the mirror and try to unwrap the UV I get this  

Notice how the mouth is different on one side, I don't understand it as it should be exactly the same. Everything else unwraps perfectly, and the only thing I did was work on that part of the mouth with the mirror invisible so I could see better from the inside. But I've looked at it and  it is definitely the exact same mesh on both sides. I've tested it a bunch of times now, same result every time. Mark seams, apply mirror, unwrap, and the mouth area is always wrong. Any ideas to why this would happen?
I had this same issue and it was doing my head in, but it turned out because the mouth and face were separate materials, the pinning of the face were messing with the mouth.