When your fiance introduces you to people as his special friend means there is something he is hiding from you. He does not want people to know that he is engaged yet. You should talk to him about it and ask him why he does so. Let him give you his story then you can decide what to do next.
Yeah its an issue you'll need to address. It be one thing if was calling you his girlfriend. I take that title as an equivalent of a friend with benefits. This is just my way of thinking. Could be wrong but how one introduces you or the title they give you does matter. It has meaning behind it.
To me it would mean it was time to say good bye as I would find it very insulting.Two things would pass from my mind,he is either ashamed of me for some reason or there is someone else.Either way it's time to kick him to the curb because if you get married what will he introduce you as..his sister?