I mean attempting to demonstrate something broken, only for it to work properly.

Wouldn't this term still apply? If the goal of your demonstration is to show failure, and then that succeeds, that makes your demonstration a failure.

That term definitely applies to any demo that behaves differently than previously observed - subtly implying that the demo setting has something to do with it.

I don't how it's used in German, but based on other comments, it does sound like it can be used either way!

As already said: Vorführeffekt is neutral and could mean both: success and failure. But the opposite of „a performance with an opposite than expected outcome“ is something that comes out exactly as it’s supposed to be without intervention, right? ;)