Well, as I am German, I can't imagine it either. But it is a fact that China doesn't allow a dual citizenship as far as I know. So once we get to that checkpoint and they see
"oh, she is German, but she is actually Taiwanese in our system. Lets find out what this is about, since even Taiwanese need a special Travel Permit when travelling to China. But now that she has another citizenship, she would acutally have to resign her "chinese citizenship" since we don't recognize Taiwan as country. So now we are going to find out if she resigned her previous citizenship, if not...."

Like I said, I have no idea if they would react that way, but I just wanted to get some information beforehand. But as I can see, there will definitely be another checkpoint so I suppose we won't book this flight.