Subjective worlds must be causally closed; they cannot transmit information to other worlds since that would break the principle that each world is subjective. As such, there can be no subjective reality: if subjective reality were true, causal closure would demand that your observations about the world are semantically equivalent to a world in which you are alone as an observer.

For example, imagine that we lived in Minecraft, which has a procedurally generated world. We use a seed (say "404") to kick off the algorithm. But say instead that we use a seed and the username of the player. Is World 404 different for each user?

No; there is no "World 404", there is World 404-TheatreAddict and World 404-Hyena and so on. The reason is that the function we use to generate the world takes two variables, seed and username, to work. A world can only be defined by this pair and has no existence independent of it. Naturally, this world is causally closed; my playing does not effect yours, you will never see my buildings nor will you ever send me angry messages about burning down 16 chunks of forest.

While this world is trivially objective, all objections to a subjectivity thesis are trivial because it is trivially untrue or unprovable. Every attempt to prove it will require a condition, like causal closure, which collapses it to a special case of objective reality. The simplest of these is the resolution to the subjective proposition itself, independent of worlds: if everything is subjective, that principle itself is objective.