Firstly, I think it is rather poor that one is not informed about this hidden cost (not that I was looking for it, but if there it should be obvious), and secondly, the words "arrest this man who is stealing from the Duke" come to mind.
No one was informed about it because it's not true. Or if it is, it's definitely a bug.
Can you provide some proof? As in, not just "I swear I remember having X bonds and Y gold, but when I converted them I ended up with X + Y - 2", but actual before and after screenshots?