That's a poorly implemented universe. Unless information propagates, it didn't happen ;-)

You could always fast forward the current state when the first external observer arrives. But I get there are time constraints - you can't just stop the real universe until it converges to a consensual state.

This does bother me about the whole "likely we are living in the matrix" thing. If we want to design a universe that does not waste a lot of processing power we need to do things like have a speed of light limitation (keep it fairly slow too). But the sensible idea is to only process lazily - don't calculate till the observer observes - but it is very much like a dependency hell.

I think they need to implement a tree tree.

So if a tree falls in a forest and no one is around to hear it, it won't hit the sound code or the display code.