It looks like there is an extra space between your ) and your [.

in this thread.

But yeah the preview thing is indeed a bug.

There seem to be a difference between preview and actual in signature as well.

Here it is in signature preview, after applying changes and reloading the page :

And here it is, same one, in a post actually :

This could have happened when they made changes over the last few days to the way forum signatures were parsed/rendered, I guess.