Same quantum experiment, different results in identical universes? [duplicate]

“If we know the complete quantum parameters, why can't we still determine the exact outcome of a single measurement?” A deeper version of the question would be: “Even with full knowledge of the quantum state, including every physically relevant parameter, why does that not determine a unique outcome for an individual measurement? Is the limitation merely epistemic, meaning we lack access to some hidden data, or is it ontological, meaning the physical state itself does not contain a predetermined value for that outcome?” If someone claims there is a deeper, deterministic theory, what would actually have to change in experiments so that it becomes testable? And if no such change is possible, does that claim move beyond physics into philosophy? And finally, if outcomes are fundamentally indeterminate, what is the best way to talk about cause in a probabilistic world?