Oh hang on, I re-read your message.
Actually, this is in fact the correct behaviour (currently) for the plugin. The text and/or theme of the button can be customised for every post or page where it's used. When you save a post or page, the code to produce that particular button is saved with the page, including any default settings.
There's currently no way of knowing which buttons have the default settings or which are 'custom'. That means that if you change the default settings it won't change the buttons you're already created as they've already been saved with the values of the defaults you used when you saved the post!
I hope this is a clear enough explanation! Basically, when I see it like this I think it's probably not the best behaviour and I can totally understand why it seems like a bug. I probably is really!!
I'll mark this as an important feature for the next upgrade. Thanks for the feedback, I don't think I would have thought of this situation otherwise :)
Have a nice weekend ~ Richard